1974 and 1975 MX Montesas both had 73M prefixes for frame and motor. The earlier frame (74 aka late VR) had conventional upright shocks, whereas the 75 (aka V75, then VA etc) had laid down shocks. All 75 and later 250 Enduros had 54M prefixes and laid down shocks. The previous enduro model from Montesa (73-74) was called an Automix and had 44M prefix for frame and motor. It had upright shocks among many other differences to earlier and later models.
VR motocross and 44M Automix enduro frames are the latest that are Pre 75 legal, anything with laid down shocks is 75 or later and not Pre 75 legal or carry over either.
