After lots more polishing the frame now looks like this:
It'll get another going over before I build it up.
Just waiting on XT M732 chainset and qr's plus a bar and stem to arrive. Sort the forks and then its build time
No numbers at all on the dropout. It did have the remnants of a very small rectangular sticker under the top tube in the extra triangle, could read anything left on it though and removed when polishing. It's an odd one.
Thanks for the comment phatmar. Those forks and wheels are going on an Orange Vit T. Sticking with RC35's and their Magura specific mounts on this GT if I can sort new elastomers.