I’ve had five Sprint Specials, and also rode the ex-John Hartshorne, ex-John Griffith, ex-Doug Wright bike (now with Martin Heckscher) quite a bit. Looking down on the tanks from above, some were the same shape as a Super Squirrel tank, and some were egg-shaped. Looking at them from the side some had parallel seams, and others were tapered. As the bikes were made on a bespoke basis by the Competition Department, for different sports, as well as fast road work, it is not surprising that they vary so much. Then A E Reynolds made his own Sprint tanks too, being ready and willing to make up a Sprint Special from the single-downtube framed 1931-1933 Flying Squirrel Sports Model. Many original tanks rotted out over the years, and replacements have been made by a number of willing folk. Jack Butterworth made me one years ago, using a pair of stainless steel pressings made by another member. No wonder there is so much variation.
If you get one that has rotted through at the bottom, they are usually firmly stuck on the seat tube with more corrosion, being virtually impossible to remove without cutting them to pieces. Plenty of grease on the seat tube when fitting a tank !
I recently supplied John Rose with a John Underhill drawing for Super Squirrel oval tanks, which could no doubt also be used for Sprint Special tanks with minor alterations. I don’t know if John has managed to locate the elusive press tool required to make the tops and bottoms, but if anyone needs a tank he would be the first person to try.