1) the little flat underneath may relate to the 65's removable recoil lock piece
Good thinking Mike. That might be it, to make room for the recoil attachment plate.
Here is one of the sleeves cut down for the FWB65 'shortie':
Vintage Airguns Gallery
..Above link posted with permission from Gareth W-B In British slang an anorak is a person who has a very strong interest in niche subjects.