You will almost certainly need to have the part made & fitted by a qualified double-gun oriented gunsmith. Probably won't cost too much and he could check it out for any other issues while in the shop.
There is probably something stamped/engraved on the top rib &/or sides of the barrels which would give an indication of either the maker or retailer among other things. There may also be something in the sides of the action or on the trigger guard.
The stamps on the bottom of the barrel flats as well as inside the watertable (flat interior of action where barrels fit) will show proofmarks and likely other information that can help identify the maker, country of origin and possibly date of manufacture.
FWIW.
