Thanks for the replies - and the link.
Sadly, the studs were definitely not still in the v brakes having checked, re-checked and triple checked very carefully such was my initial confusion!
They were fitted, simply with extra long bolts (much longer than a standard canti would use), and done up tight, but the v-brakes still had full movement. An M730 canti, when fitted in the same way, would not move... The v-brakes were simply rotating around the bolt with no sleeve at all. Clearly, the previous owner has lost them over time and simply had this running as was.
However, much much thanks to the kind RetroBike user that has offered to send me a pair of these collars - so very much appreciated! Am very glad to have a solution that doesn't involving mashing up the originality of the frame simply to get the brakes to work! Now, to choose a colour scheme that's going to look awesome......
Rossin Concept MTB 1990 - Full XTR M900
Litespeed Ocoee Titanium MTB 1992 - Grafton / Bullseye / Syncros / XTR
LeMond Z Team Issue SLX 1991 - Full Corsa Record / Sigma
LeMond GAN Team Issue GLX 1993 - Full Dura Ace (7400)