It's a Model 600, with 18.5" barrel. The Model 660 was the successor version, with 20" barrel sans the vented rib. More traditional looking, whereas the M600 looked like an alien plasma-rifle "in der 40-Vatt range."
I didn't. Maybe my comment on recoil in the main thread was confusing. The M600 Magnum Carbine pictured is an original configuration (unmolested) specimen in .350RM, not .404J. The M600 is the grail gun I've been chasing for years.
Having shot a few M600s owned by other folks, I was just saying that the felt-recoil of my AHR CZ 550 in 404J with a 23" barrel was worse than a M600 in .350RM with a 19" barrel. At least that was my experience.