This gun is unique, in that it is chambered for the 16 gauge shell and stamped U.S. property. I have been told that these guns were used to train Naval gunners. My attempts at researching this gun always result in frustration as I always find conflicting data.
Today is December 29, 2019. And as I'm updating this webpage, I decide to take another run at finding information about this one. Again it led to frustration, so I broke down and ordered Browning Auto-5 Shotguns, The Belgian FN Production - Revised, Second Edition.
Update: January 3, 2020 - My book came in and it appears the 16 gauge gun was likey made in 1928.
If I were shown this photo and asked to guess which one is stamped U.S., without a doubt I would guess the shorter 12 gauge, but I would be wrong.
Note: Regarding the Serial Number Table for the Pre-WWII 12 ga. guns 1924 - 1939, see this page for 12 ga. Serial Numbers.
In my online research today, I found that Browning offers Historical Letters that include:
All letters come on official Browning Historic Letterhead which is authentically stamped and enclosed in a plastic cover. Contact: 1-800-333-3288