A bleep in his headphones led to Colin Pearson digging up the coin pictured here. It’s a hammered silver groat of Henry VI, which is a fairly common type but it’s in very good condition.
The annulets on both sides of this coin point towards it being struck during the annulet coinage, which commenced in 1422 and ended circa 1430. The inner legend on the reverse is VIL LA CALI SIE so this groat was struck at Calais, which at the time was an English possession.
The reverse would grade VF+ and has been struck with great care. The obverse isn’t quite as good as the king’s hair at the left hand side is weak. If sold at auction a Henry VI groat as nice as this one should achieve a hammer price of at least £150.