Wonderful gun Mark.
If you would post full size close up images of every mark on the barrel flats and just forward of the flats we might be able to date the gun.
OWD might be correct, but there are lots of A&F and a few Francotte catalog repros available.
You've seen this?
https://www.shotgunworld.com/threads/abercrombie-fitch-von-lengerke-detmold-and-francotte.393236/
Francotte imported to the U.S. usually had the chamber length marked in inches.