Hi. looks like it was proofed under the Rules of 1925, used until 1954. The left barrel is choked, the right cylinder, or just a touch of choke. The 13 is the bore diameter. If the gun was a true 12b of .729 then it would have been stamped 12. The gun was proofed at the Birmingham Proof House
Alfred Davis had at one time been the manager for J. Blanch shop, thus the "From J. Blanch & Son" Looks like he was in business from 1933 to 1941
As to the date there should be a crossed sword mark. That will give you the date the gun was viewed, i.e. made. For your gun, there should be a letter on top and a number on the bottom of the crossed swords.
Best,
Greg