Sandon

FYI, a doubled die is classified as a die variety, not a mint error. A die variety generally pertains to most or all of the coins struck by a particular die pair, while a mint error occurs to individual coins. This coin is a major die variety listed in the "Red Book" and other standard coin price guides and is worth several hundred dollars even in circulated grades.

The die scratch from the rim above the "D" in "UNITED" is a diagnostic for authenticating the "Red Book" variety 1972 doubled die cent, as the OP apparently knew.

Welcome to the NGC chat board. Unfortunately, your 1983 cent is an example of a coin that was altered after it left the mint to superficially resemble a coin that had been multiple struck. The secondary image being backwards, incuse and having a date (1976) from some years earlier reveals it to have been squeezed between two other struck coins, likely in a vise. See https://www.error-ref.com/squeeze-job-or-vice-job-or-garage-job/. Compare this with images of coins that were actually double struck at https://www.error-ref.com/multiple_strikes_on-center_off-center/, https://www.error-ref.com/mutiple-strike-in-out-collar/, https://www.error-ref.com/mulitple-strikes-flip-over/. Mint errors are an advanced topic in numismatics (coin collecting). It is very unusual to find any coin with a significant mint error in circulation. Unless you have some knowledge about coins, including how they are made, it is easy to be deceived by fabrications like this one. If you are interested in learning about U.S. coins and how to collect them, please so advise, and we can direct you to appropriate print and online resources.

Welcome to the NGC chat board. Unfortunately, what you have is a crude counterfeit of a 1799 Draped Bust silver dollar. Large numbers of such bogus pieces have been produced overseas in recent decades, and some are older than that. For comparison, here are images of a genuine example, now graded VF 35 by NGC:

When you post coins on this forum, we assume that you are a newer or casual collector who is trying to obtain other opinions about them. You don't have to agree with any or all of us, nor should you take offense if some or all of us disagree with you. We have no way of knowing what you do or don't already know about coins, and when I include links to pertinent resources, I do so because I think that the topic author and/or other readers may find them helpful. We ask for full cropped photos of each side of a coin to assist us in evaluating it. We've never received such photos regarding the 2005-D cent. Based on the close-ups provided, your coin doesn't appear to me to exhibit die doubling on either side, nor does the darkened area appear to me be matter that was struck into the coin. Regarding the doubling, you might want to check such sites as doubleddie.com or varietyvista.com that list more minor doubled die varieties to see if you can find a match. Although foreign matter that was on a die or planchet has been embedded into coins at the time of striking, neither the 2005-D cent nor the 2021 Silver Eagle appear to me to be examples of such "retained strike throughs" based on the photos provided. See https://www.error-ref.com/?s=retained+struck+through.

The plating of copper-plated zinc cents is supposed to be pure copper, and whatever amount of zinc is in the plating--possibly more than the 5% that was used in the homogeneous alloy of both 1944-46 and 1962-82 brass cents--gives the coins a noticeably different color, which some collectors apparently find significant.

If the coin is determined to have been "cleaned"--which supposedly would have occurred before the coin was "conserved"--the coin will be given an adjectival "Details" grade, with "Cleaned" noted on the label. The coin's having been "conserved" is not noted, and the coin is supposed to be given a numerical grade if found to be unimpaired. However, I do not recommend allowing NCS to "conserve" coins based upon my one experience with it.

You are apparently referring to a 1982 or later copper-plated zinc cent whose copper plating had been contaminated with zinc, resulting in the plating having a yellowish color referred to as "brassy plating". See https://www.error-ref.com/?s=brass+plating for a full explanation. I don't find this slight color difference of much interest either, but there are apparently enough collectors who do to make grading services consider it worthy of attribution.

Welcome to the NGC chat board. You do appear to have found an actual "Red Book" variety (FS-101) 1972 doubled die cent. If it is submitted to a third-party grading service, it would likely receive a "Details" grade due to the deep scrape in Lincoln's head, but it may be worthwhile doing so for proof of its authenticity. NGC does not require the $18 VarietyPlus fee for this major variety. It would be less expensive to do so if submitted with other coins or through a dealer. You could also just keep it in an album or holder with a notation as to its variety or significance.

I don't know why the coin couldn't be entered into your coin list and used in a custom set, but if you are trying to add it to a competitive set, coins certified as mint errors aren't eligible for competitive sets. The Administrator explained this policy as follows: Coins designated/labeled as "MINT ERROR" are not eligible for competitive NGC Registry sets. When our graders (or PCGS graders) designate something as a MINT ERROR, as a rule, these are ineligible for the NGC Registry. Each piece is rare in its own way. Mint errors are unique, and prices can vary greatly and are highly subjective. Therefore, to allow them in the NGC Competitive Registry would be unfair practice. The exception to this rule is the US Presidential Dollar - Edge Lettering error NGC Registry set; these coins are more common and are similar; as such, we can assign them a competitive registry score and require others to obtain similar coins to fill their registry set. Therefore, competitive sets for these PEL error coins are the exception to the rule. You may enter your ME coins into Custom NGC Registry sets, if you wish.

You should post questions like this in the "Newbie Coin Collecting Questions" forum, as the NGC staff does not provide such information. If you are asking in regard to a specific coin, please provide clear, cropped photos of both sides of the coin. I'm unclear whether you are asking about a coin that was struck twice, which is classified as a mint error, or a coin struck from a doubled die, which is classified as a die variety. For the former, see the examples of "double, triple, and multi-struck coins" at Learn Grading: What Is a Mint Error? — Part 2 | NGC (ngccoin.com). For the latter, see Double Dies vs. Machine Doubling | NGC (ngccoin.com). Both types of coins, if genuine, have collector value that depends on a number of factors. Regarding the latter, there are forms of "doubling" on coins that do not result from doubled dies and generally have no value, one of which is the machine (a.k.a. strike or mechanical) doubling referred to in the second link.

1890-S Liberty Seated dime, NGC graded AU 58:

1839 Liberty Seated dime, now NGC graded AU details, cleaned:

Unfortunately, the photos are too blurry to determine whether this 1955 cent exhibits doubling of any kind. If you are unable to provide clearer photos, I cannot render an opinion as to whether this coin was struck from a doubled die. You may want to check NGC VarietyPlus, PCGS Coinfacts, doubleddie.com, and varietyvista.com to see if you can find a match. Please be advised that NGC only attributes varieties that are listed on VarietyPlus. See What is a Variety? | NGC (ngccoin.com). Apart from the major (FS-101) variety, the only 1955 DDO variety that NGC attributes is FS-102. See Lincoln Cents, Wheat Reverse (1909-1958) | VarietyPlus® | NGC (ngccoin.com). I understand that PCGS only attributes Lincoln cent varieties listed in the Fivaz-Stanton book, The Cherrypickers Guide to Rare Die Varieties. See https://www.pcgs.com/varietyfaq. ANACS may attribute more obscure varieties, but it is doubtful that such varieties would have substantial market value. If your coin is an FS-102, you may wish to check auction records for this variety before deciding whether to submit this circulated example. Although you would learn by submitting the coin without taking these steps, I don't think that you should "learn the hard way", which is what I think @JKK means.

Welcome to the NGC chat board. Your purported 1794 silver dollars are obvious counterfeits, likely of the sort that have been mass produced in China over the past few decades. See 1794 BB-1,B-1 $1 MS | Coin Explorer | NGC (ngccoin.com) for photos of a genuine example of this rare coin, of which fewer than 200 are believed to exist. All of these coins were struck from the same pair of ties, so the shapes and positions of all letters, numbers, stars and other design elements are identical on all genuine pieces. Note, just for example, the differences in the appearance of Liberty's face and hair and of the eagle on the genuine piece from the counterfeits.