Thanks GreenMt for putting me on the right track. After further research I’ve discovered the following. The JD mark is for Jean Duchateau, a gun maker (1908-1964) in Liege that made everything from the finest firearms to knock about field grade. Sold under many names.
The 18.4 mm mark is for roughly 12 gauge.
L with star above is for Controller of Proof Codes Salmon Antoine who served from 1937 to 1968.
There is an actual Belgian year mark that looks like a “u” with a moon above it but I can’t match it to any of the script letters used during the period.
From what I’ve read, the Belgian gun makers didn’t retool for external hammers after WWII, if that is true the date is somewhere between 1937 and the end of the war, maybe. On the other hand they may have had many parts left from prior years and used them up for trade guns, or other, maybe?
Any thoughts from members of the forum? Concur, non-concur? Other?
-gary