As far as I've found there is no way to pin point date them, info just shows made from 1914 to early 1950's then went to the side load SA model in the mid 50's.
Early ones just like the FN Trombone had a screw elevator, real early was a one piece leaf finger screw on the Trombone later the dovetail was a separate piece and screw took a small flat blade, then next change had a thinner leaf and a step elevator. I've seen the same on the FN semi minus the one piece leaf, personally have only have seen them on a square tang Trombone, so uncertain about that.
Sorry to mix the two models but they were from around the same period from the same factory both Browning designs.