D = 70 mm = 2 3/4" chamber at 20.7 mm = .815" = 12g
The pre-fit and finish barrel weight was about 1.34 Kg = 2.95 #
12805 was the maker's SN. No records exist to date the gun further.
Please see
http://www.shotgunworld.com/bbs/viewtopic.php?f=5&t=366087 The gun was proved for a (pre-1924) service (using) load of 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps).
That was likely about 100 years ago, bad things happen to guns and the head of the stock in 100 years, the gun deserves a careful check and clean by a double gun specialist smith, and I would NOT use that load now.