Antique 18 Century British English London Marked Pewter Plate For Sale

This looks like a plain rimmed 18th century plate.On the back there is a triad of letters M over JM. This format was common in England for a marriage triad where the man with surname beginning with M and Christian name beginning with J married a woman with Chistian name beginning with M.The other marks are largely undecipherable.It appears to have what looks like a Britannia mark.