The famous Panama Pacific Exposition half dollar without the "S" mintmark struck in gold.
These strikings, before the mintmark was added, were probably deliberately struck to create a rarity. Pollock mentions a quote from Breen by Farran Zerbe which more or less describes them as pieces struck for the Secretary of the Treasury.
Examples are known as follow:
Gold J1960/P2031. This used to be J1793 in earlier editions. Only 2 are known:
1) Col Green, Olsen, Farouk, Norweb, Queller-Heritage 1/09, 2010 ANA, Simpson collection - NGC64
2) Col Green, Sloss-79 ANA, SRCA 6/81, Heritage 11/03, Southern collection, Simpson collection - NGC64 example which is illustrated above.
Silver J1961/P2029. This used to be J1791 in earlier editions. At least 4 are known:
1) Sloss-79 ANA, Heritage 11/03, Southern collection, Simpson collection - NGC64
2) Col Green, unknown owner
3) Col Green, Newman-EPNNES, Heritage 4/13 - NGC65
4) Col Green, Newman-EPNNES, Heritage 11/13 as NGC64, Heritage 8/15 ANA - PCGS64
These last 2 examples appear on page 86 of the inventory journals of Eric P. Newman and B.G. Johnson's purchases from the Col. Green collection circa 1940 as shown below.
Image courtesy of Eric P. Newman, Stuart Levine and Len Augsburger.
Note: Eric Newman's hand written notes show that Col Green owned both gold, 4 silver and 3 copper examples.
Copper J1962/P2030. This used to be J1792 in earlier editions. The following are noted.
1) Sloss-79 ANA, Heritage 11/03, Southern collection, Simpson collection - NGC65RB
2) Heritage 96 ANA - PCGS63BN, same as #1 ??
3) Newman-EPNNES, Heritage 4/13,Simpson collection - NGC66RB
4) Swiatek, Heritage 1/15 FUN as NGC66RB, Legend Auctions 2/16, StacksBowers 8/16 ANA, Legend Auctions 7/17 - PCGS67RB
Photo courtesy of Heritage.