It's a Trackmaster Frame, they were basically the model for the 72 on Oil in Frame BSA and Triumph and very similar in many respects to the XS frame. Basic single backbone, twin down tube, twin shock frame.The tail wrap of the frame looks kinda like an XS
Like the bobbed look of the fender too
Pondering
A fender support loop on a XS frame would give you a very retro bratish look. While true Bates seats are pricey anymore, there are multitudes of knock off combos at the various chopper suppliers and of course e-bay. I'm building a vintage track bike based on a BSA 250 single and have purchased a similar seat combo there pretty reasonable. There's all kind of options for color and pattern of upholstery if you want to alter it some.