A contemporary forgery* of a late medieval English penny.
The die-cutter's lack of skill is evident in the standard of engraving, and the legends have been completely flattened, possibly to hide his illiteracy. The coin appears to be made of reasonably good silver, but is considerably underweight, possibly an indication that the raw material was sourced from clippings taken from genuine coins.
The coin seemingly imitates a York penny of the mid 14th to mid 15th century, as the quatrefoil on the reverse has a pellet in the centre, but it isn't possible to attribute it to a particular reign. The obverse, with saltires by the crown and an annulet on the breast, is not a combination found on any official coin. An annulet on the breast, alone, occurs on various York pennies of the fourth coinage of Edward III, and light coinage pennies of Henry IV, but the saltires were possibly inspired by a leaf-mascle issue penny of Henry VI (Spink 1867).
Certain Irish pennies of Edward IV have a similar reverse type with a pelleted quatrefoil at the centre, but none have been traced with the obverse marks of the present coin.
* An alternative possibility is that the coin is a severely mis-struck York penny of the light coinage of Henry IV (Spink 1734). This could account for the misalignment of the cross and elements of the quatrefoil on the reverse, and the apparent marks by the crown on the obverse.