Actually Martin, it does not have a ‘long tank’. On the Sprint Special these were of a similar shape to the tank on a vintage, duplex frame Flyer, ie. the slim type with parallel sides, but on the S.S. they had a totally different way of fixing, with a clamp around the downtube at the rear. They also had different tank caps, being fitted with the lovely CMC (Coventry Movement Company) caps, with a tommy-bar locking action. They are more usually seen on Brough Superiors, but my 1929 Senior TT works bike has them as well.
The S.S. in the Bonhams auction, in fact has a 1931 to 1933 Flyer type tank, where the gearchange gate bolts to the tank, rather than to the frame. Different shape altogether to the S.S. ‘long’ tank, and ordinary bayonet type filler caps.
Brian
