The "experts" mentioned in Mr. Gibson's letter were likely of similar types we encounter even today. These are what I call the "Looks Like" experts. Back in 1889 it was "It looks like a genuine nickel," today this type says "Looks like a branch mint proof." In both instances there was not enough background knowledge for the "expert" to make a meaningful assessment, but the "expert" didn't understand this.
In the 1889 reply, a mint officer suggested looking at the edge for a soldered seam, along with weight, diameter and ring. (Nickels didn't have much of a ring, but a glue or solder job would simply go "thud.")