It's a No. 00. Unfortunately that SN is missing from the remaining shipping records but it likely dates to about 1899.
Yes, the forend is a later replacement.
The pad looks to be a One Layer Jostam Sponge Rubber



Please looks through the FAQs for lots of information. The gun certainly needs a check and clean by a qualified double gun specialist.

BTW: I shoot recreational skeet weekly with my well used 1906 No. 00