Thanks for the pictures, Mike. That is indeed the flat top strap. That supports the reported transition from the rounded top strap in 1922. I have become very interested in the evolution of the 20's era guns lately. Dating this period is tricky because guns were shipping,in some cases, years after they rolled off the assembly line, and not in serial number sequence. Somehow DWalt is never too far off on dates.
You'll notice that the trigger is missing the semi-circular slot for the second pin on the hand, but as far as I can tell, it's the same as later triggers otherwise. A later trigger will work in this gun, but this trigger won't work in a later gun.