Tom Doniphon And fredh46, I have been researching all of the US combat shotguns, primarily the trench shotguns from WW1 and WW2 for over 3 years now. All the information that I have and have seen (Complete Guide to United Statesicon Military Combat Shotguns - Bruce Canfield & The Worlds Fighting Shotguns - Thomas F. Swarengen being the largest information sources), there seems to be a lot of information that is missing. In reference to the 1943 serial number, I think what you will find is that NO PARTS (receivers, barrels, ect.) were produced but there were still preexisting part left to put together M-37's till production started up again. I am sure this is the case because I have seen this before, including a Lee Enfield No.5 Mk.1 that was marked 12/47 even though production was halted in 6/47, there were part that made it possible to put together No.5's till 12/47.

As for the comment of "no "civilian" shotguns were produced by Ithaca during the war", Winchester, Remington, Stevens, and Ithica never produced a dedicated "military" shotgun, they were all built on civilian produced shotguns. They only major change was a heavier barrel that had thicker walls and of course, the fact that before being issued they were stamped and given martial markings. If you are wondering if it is a Trench gun versus a riot gun, I seem to recall that the Riot guns didn't receive the heavier barrels. The trench guns had the heavier barrels and had both ordnance stamps as well as trans versing slots cut under the barrel. Unfortunately, my copy of Canfields book is with a friend right now, but I do remember the approximate number of m-37 trench shotguns is between 1600-1700. 1678 comes to mind.