The 600/660 came along long after Rem changed the twist rate...twist rate was changed from 12" to 9-1/4" when the 700 was intro'd in '62 and the name was changed to 6mm from 244, although there are reports of some 700's leaving the factory with 12" twist barrels...probably pulled from the 722 barrel pile.
You're right, the twist was changed with the intro of the M700, but (I think) the name didn't change to 6mm Rem until 1964?.......
Casey
