Good shout, chaps. It does look like a silver hallmark on the plaque with a *makers mark first, then the anchor for Birmingham, lion passant, and the date letter B for 1951. Edit: the style of cartouche is correct for that year too.
*The makers mark could be JD, JO, JQ...?
Also, what a wonderful story...and best thread conclusion ever!
Edit: I'm no expert on British silver hallmarks but, to my untrained eye, the hallmark date of 1951 predating the actual event in 1952 is entirely correct, the silver plaque would have to have been made and assayed at Birmingham before sale to BSA for mounting in the stock. The hallmark must always predate any engraving...