There are several 'JM's here
http://www.littlegun.be/arme%20belge/a%20a%20artisans%20identifies%20gb.htm#MPete Mikalajunas' maker's mark list had 'JM' as Joseph Monisse. I have no infro.
But the mark is likely that of the Stub Twist barrel makers Joseph et Jacques Moray of Nessonvaux. "Fine Twist" clearly indicates the gun was for the English speaking market.
Inspector's marks were spangled after 1877 so the gun dates before then
14 bore is .693". 13 bore is .710"
This is the earliest Belgian Proof House document I've found - in English!
Monthly Consular and Trade Reports, “Report by Consul Robertson on the Manufacture of Fire-Arms in the District of Verviers and Liege – (1853) Rules of the Proof House, Gun-Proof Establishment at Liege, Belgium”, 1885
https://books.google.com/books?id=7EhJAAAAMAAJ&pg=PA142&lpgA standard c. 1880 "Light" 12b English load was 2 3/4 Dram Curtis & Harvey No. 5 Black Powder with 1 oz. shot
