Exchange, sale, purchase and other distribution of pattern/experimental pieces was commonplace during the 19th century. The Newlin exchanges during 1885-1886 merely happens to be clearer than most.
The last attempt to confiscate pattern/experimental pieces was in 1910 by Director Andrew. It failed, as have all others. The difficulty is that many patterns were legitimately sold on authority of the Mint Director - an authority which he/she possessed. Further, none of them were coins - the design or composition not having been legally adopted. Pieces given to members of Congress or others, but not accompanied by a return order, were considered nominal gifts of valueless chattel. That is, the US Mint/Treasury had to establish that it was retaining all ownership in the pieces. A more recent circumstance reinforces this. At several of the Annual Mint Conferences held in Washington and Philadelphia, the Mint and/or BEP have given - free and clear - sample planchets, autographed currency and other items to participants. There was no accompanying statement retaining ownership, so the items were legal gifts.
Decisions precluding items called patterns or experimental pieces from being confiscated date from the Linderman auction and Director Kimball's attempt to seize certain pieces as being illegal off-metal items. The US Attorney disagreed with Kimball's arguments and the pieces were allowed to be retained by Linderman's Estate. Andrew's 1910 attempt, which included a "sting" operation, failed for similar reasons and affirmed that "once the horse was out of the barn, it was free."
A parallel conclusion was reached in regard to US gold coins that had not been subject to distribution by common means. Treasury counsel determined, with support from Mr. Tripp and myself, that if any record or any kind existed showing even one piece had been released on authority of the Treasurer or other Official (including the Director), then all such pieces were deemed legitimate. DE of 1933 were the only coins for which no release record could be found. (The Farouk export license could have been argued as applying. This was explained.)