The rear end of a vintage Flying Squirrel (parallel sides) fits into the TOP of the saddle mounting lug, in a slot. When the duplex frame was reintroduced, in 1934, the tanks were all flared sides type, and the rear end of them fits into the UNDERSIDE of the saddle lug, the top of which is not slotted. In addition the duplex frame geometry changed for 1934 onwards. A post-1934 tank will not fit a 1927 to 1930 frame, although a lot of bodgery has occured when owners have tried ! I do have an unusual large-capacity vintage tank that seems to be an interim development from the ‘slim’ parallel sided vintage tank and the Rep type tank, but I’ve never seen another one like it. (Not for sale !)
