Chris sorry I have no idea, but am trying to learn something
so I'm guessing the initials would be the engravers initials and given it says 'owl' it may be an British engraver?
I do know that somewhere there is a list of engravers or an engravers society? maybe a google trawl will bring it up. The only JM I know of is Jonathan Matcham and that is only because I have a book written by him and Peter Dreiser on Glass engraving. However there is nothing I can see similar to your owl in there. I bought the book because it has plates of some of the most amazing engraving, truly breathtaking - despite that I am not into engraved glass I would love to own one of those pieces
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