It's D.A. 6665. 6665 is stamped on the water table, flats, both triggers, the floor plate, and possibly other parts I haven't gotten to yet. So I'm assuming that's the serial number or at least a number to prevent mixing of parts since everything seems to be hand-fitted to some degree. The only place the D.A. appears before the number is there on the barrels. D.A. may be the maker's initials?

I have some digital calipers for my reloading bench, would that work for determining the choke?