Re:
They where adjustable for vbrake modulation, same as modern day ones are (even up to XTR)
If it suits you on canti's then that's fine.
They're horrible levers anyway.
Talking of the M600 and similar.
There where earlier canti's levers. They where not separates iirc like all the other Shimano levers of that era.
They only separated again as Vbrake levers.