When my gun was checked in 2004 the holder of Lancaster records was:

Mrs. M. Mitchell
19 Portman Crescent
South Bourne, Bournsmouth, BH5 2ER

I wonder if the under lever/cocking lever is a replacement. The rest of the gun is so well engraved and fitted, that the lever seems out of place.

Good luck with your search. I have phone numbers (ca 2004), but understand that a letter is best.

Joe