It's about old english barrel I assume.
There is also proofmarks on the barrels like:
"best english martin's siemens steel", "18739", "RP" - under crown, "nitro proof", "choke 1 1/4", "12 over C" - in diamond, "BP, NP, BV" - under crowns...
It's cal. 12/65-70 (not sure), barrel length 70.3 cm, break in neck on left side, hammerless.
I can send some pictures too if it's needed.
Any information is welcome.

"Each problem that I solved became a rule which served afterwards to solve other problems."