Are there besides seat lock any other differences between MK2 and MK3 frames?
Look for a 6-digit number stamp on the front face of the LH fuel tank mounting bracket. This is the (850) frame
part number.
The early 850 frame part number is 064140
Later pre-Mk3 frame is 065404
Mk3 frame is 065632.
Andover Norton Parts are the only Norton spare parts in the market manufactured to original drawings, within the factory-approved tolerances, from the correct materials, and with the correct finish.
andover-norton.co.uk
A Mk3 frame has two 06.5620 brackets. One is the
left-hand side outer shock mounting plate (for seat hinge attachment) and the other is the
right-hand side inner shock mounting plate (for the seat lock).
Also:
LH inner is 06.6102.
Steering lock plate 06.5422 although Andover Norton also supplies this as a replacement for the pre-Mk3 frame steering lock.
https://andover-norton.co.uk/en/shop-drawing/462/1971-on-commando-frame-repair-items
A pre-Mk3 frame would normally have either four 06.0412 plates...
FRAME R/SUSP
andover-norton.co.uk
...or two 06.0412 outer plates and two 06.4282 inner plates.
https://andover-norton.co.uk/en/shop-details/20497/frame-r-susp-mount-plate
I don't know the differences between MK2 and MK2a
The main differences are that the Mk2 (the US model) had the metal perforated airbox, peashooter, or Interstate silencers/mufflers, battery in line with the frame, LH side panel secured to the frame at the front by a metal Dzus fastener and previous style kickstart lever.
The Mk2A (European low noise model and also a limited number of Mk1As) had the black plastic airbox, 'black cap'/'bean can' silencers (often replaced with peashooters), battery positioned across the frame (due to the larger size airbox) LH side panel secured by a plastic Dzus fastener at the lower rear of the panel and kickstart lever angled to clear the black cap silencers.
Some Mk2As were also sold in the US in late '74.
No.