Watch List notes:

The Hardcastle Collection | EXTREMELY FINE 40 [Top Pop] | George III, Emergency Issues, Countermark Half-Dollar, oval counterstamp on Charles III 4-Réales, 1781-PR, Potosi (ESC 1880 [611] R2; S.3767), the host coin near very fine, the countermark good very fine, rare, in NGC holder, graded XF40 ~ C/S UNC Strong (Cert. #5878623-001) [*Single Finest*].
https://www.ngccoin.com/certlookup/5878623-001/40/
Estimate: £400 - £500