This is one of 2 known Great Britain Crowns (1977 Silver Jubilee of Reign) that was fully indented by a smaller planchet. A 1 Pence planchet entered the collar and rested on top of the Crown planchet. The obverse die struck the 1 Pence planchet and the Crown planchet simultaneously.

Crown & Dollar sized coins that were fully indented by a smaller planchet are rare. This fully indented Crown is part of a mated pair. It is possible that the other coin of this mint error pair was released into circulation as well. It would be a 1977 Great Britain Crown struck on a 1 Pence planchet, and the reverse would be uniface.

Indents are featured in
my NLG Award winning book,
World's Greatest Mint Errors.