This hammered silver coin was found by John Lashmar, who is a regular contributor to my website.
The coin is a voided short cross penny of King John, who was a younger brother of Richard the Lionheart. John was one of the greediest kings to ever sit on the throne of England. He oppressed his nobles as well as his people and in 1215 John was forced to sign the Magna Carta, which was meant to compel the king to rule within the established laws of the land.
On the reverse of this penny is a legend reading +WILLELM T ON LVN, so Willian T is the moneyer and London the mint. There is a T after Willelm as at the time there were other London moneyers with the same Christian name; therefore, they were they could be identified by a letter after Willelm.
On the obverse the style of the letter X in REX marks out this penny as belonging to class 5c.
The coin has some weak spots but is otherwise in better than Fine condition. To a keen collector it should be worth around £50.