The following is a breastplate form the Royal Collections trust. It states that the Helmeted A was used by the London Armourers' Company after 1649/1650. What I thought were the letters "NM" to the right of the Helmeted A is probably the top of the symbol of the Cross of St. George giving a more exact date of 1650 to 1660 if royal collections trust is correct.
https://www.rct.uk/collection/67302-...rs-breastplate