i think you may be talking about what was called a 'direct on' front mech.
they were mostly produced for road bikes, but some mtb specific versions were made.
Thanks for all your replies - all good ideas. Really need to see how a) what type they "did" use and b) what the availability of a decent alternative would be.
Im not sure where the stops are on the frame or indeed how anything can be bolted to it. All in all it will be an interesting experiment Im sure.
If it is a braze on mount, Shimano road works ok with mtb shifters. I've got a Dura Ace on the Klein working XTR stuff fine. Justs needs to be set up with some slack when in the granny ring.