Got the engine number from the other frame tonight. Just took a bit of a scrub with a wire brush and a torch to get light at the right angle to read it.
This seems to date this frame as a 75 model whereas the other one is a 74. Since this one is the one that the (as yet unfound) engine was supposedly from that probably means it is a GT750M. Will be most interested to find the engine as that will settle it once and for all. Probably be a couple of weeks before I get a chance to look for it.