This is the only known obverse or reverse die cap struck in gold. It was recently certified and authenticated by NGC. It is an English Half Sovereign dated 1824 of King George IV. This happens to be an extremely rare date in the English Half Sovereign series and is also in gem mint state condition.
After being struck, this gold coin adhered to the obverse die and became an obverse die cap. It subsequently struck several blank planchets, creating brockages on the obverse of each coin. The reverse design of this die cap expanded with each strike. As with any obverse die cap, the shape resembles a thimble or bottle cap depending on the number of strikes.
This unique gold obverse die cap has an incredible pedigree including Eliasberg. The Eliasberg coin collection is world famous and many records were set when it hit the auction block.
I consulted many of the recognized mint error experts and dealers and none of them had ever heard of another gold die cap. They were surprised that one even existed and thought it was fascinating that it was on such a rare English gold coin from the Eliasberg collection.