Watch List notes:

Half Dollar George III Octagonal Countermark on a Mexico 4 Reales 1789Mo (Mexico City) countermark VG host coin Fine, unlisted by ESC, Bull 1889 type B/2, listed as Rarity R3, with an accompanying footnote stating 'All octagonal countermarks are very rare, most on this (Bull's) list being unrecorded

Estimate: 550-650 GBP