Trackmaster project on UK eBay. Looks pukka. Buy it now, £2300
Owner claims it might be ex-Corky Keener, but doesn't have proof (and admits it), other than a sticker and that doesn't mean much.
He reckons it's for an XR750, but I don't know how much they differed from parallel chassis.
Go to Trackmaster rolling chassis
Thanks to Jon S for the tip-off