Bruce Bartelt writes:

For more information, see:
electrotype; medallion(https://www.britishmuseum.org/collection/object/C_B-11501)
The great find and great loss of Childeric's treasure(http://www.thehistoryblog.com/archives/37323)

Alan Luedeking writes:

Alan V. Weinberg writes:

Alan adds:

Circa 1964 an armed robbery at Lester Merkin's NYC office wherein numerous rare coins were stolen, coins appearing soon after in other NYC coin shops.

Circa 1966 I discovered the switching of rare American coins for more common coins in trays at the British Museum. At the time the unsupervised exhibition of coins to off the street visitors invited into their vault rooms was the practice. I discovered this while being allowed to view tray upon tray of treasures over several days. One coin missing was an Uncirculated 1792 half disme donated by either Sir Joseph or Sophia Banks circa 1800. In its place was an EF cleaned 1829 half dime. At least the switched-in coin was the same denomination. As a result of this revelation, the BM instituted considerably more secure visitation viewing.

One of the DuPont armed robbery coins was a 1787 gold Brasher Doubloon (recovered years later) but still outstanding is a unique silver 1866 no motto seated dollar.

A red 1864-L Proof Indian cent, slabbed high grade, stolen out of Rick Snow's bourse case at a Baltimore show several years ago, later traced to Canada and still outstanding. A six figure coin.

A long term switching of large cents from the NYC ANS by William H Sheldon who resold the higher grade ANS large cents with his collection, with many coins ending up in innocent 3rd party hands. Most coins recovered and returned to ANS with some cents still outstanding.The Sheldon switching extended beyond the ANS into other private collections.

To read the earlier E-Sylum article, see:
GREAT NUMISMATIC THEFTS(https://www.coinbooks.org/v24/esylum_v24n08a15.html)