Hi Dave, I think, but am not sure, that you have a tank made by A E Reynolds, or perhaps Hemmings. Certainly much easier to remove/refix ! What is your removable rear chainstay frame section like ? Is it all tubular ? Or does the bit under the saddle consist of a big cast lug incorporating mounting holes for saddle springs, with a forward projection with a hole for mounting the nose of the saddle? At the bottom end of your seat pillar tube does the lug have a square shoulder for the tank to sit on, or does it taper off ?
Brian
