Help needed to identify mark please

I have a pocket watch and the mark is a closed belt loop with Fine Silver engraved in it and the initials A.M. inside the loop. Below is a number 4213.8.

I am only used to looking at the usual Sterling marks, cities and dates etc and wondered if someone could throw some light on this piece for me and why it doesn’t follow the usual set of marks?


Although the modern definition of “fine silver” is 99.9% pure I have always understood the use of the term on old items, and particularly watch cases, to mean silver plated. 99.9% silver would be far too soft for making something like a watch case which is why silver is normally alloyed with another metal to make, for example, sterling silver at 92.5% pure. AM would presumably be an abbreviation of the maker’s name.