Object number "II.223" and "II.353 B" at the Royal armouries are breastplates bearing "Helmeted A" markings and "II.318 c" bears the "Helmeted A" and "EO" markings. All are breastplates from the Tower Arsenal and according to the Royal Armories date to 1850. The following Bonham's lot of a set of Pikeman's armor bears similar markings.
https://www.bonhams.com/auction/2102...ikemans-armor/