ZR,
The original M-600 had a lwt contour barrel and a vent rib. (18.5" lgth) Mfg from '64-'68.
The M-660 had a 20" standard contour barrel with no VR and better sights... Mfg from '68-'71.
The Mohawk 600 was was a clean-up of the parts. It had the same stock as the original M-600 with the standard weight barrel of the M-660 but only 18.5" long. These were manufactured from '71-'79.
I liked the M-660 the best. and I have one in .222 Rem, and one in .308 and another in .338 Fed. (bored from .308)
Hope this helps...
GH
