Help with identifying a vintage sterling locket mark - an M over another letter....

[roosal]

Hello - and thank you for your time & consideration! I recently purchased a pretty round vintage sterling locket at an estate sale that has a makers mark on the back (smooth) I just cannot find. It appears to be the letter M over another letter, that I can't make out. I've tried B, H, A. Can't find anything similar to this mark. Would anyone here happen to recognize the makers mark? Thanks VERY MUCH in advance!