The true RSM was never manufactured in 458 Win Mag. The RSM is a Mark II. But has an integral quarter rib machined in as part of the barrel. Pretty unique and special feature of that model. And the word is that Ruger lost money on each and every one. But built them because Bill wanted to, and could afford to. The RSM also has a few other special things about them.
The 458 Win mags I've seen are older model guns prior to the Mark II. No quarter rib. Nothing special.
Hi Bob, how's things going in Wyong?. Down your way a couple of years back but haven't been in NSW since Ebor for the fishing. just getting over some nasty storms up here in Qld, seeing the sun for the first time in a few days. I'm going to NZ in the spring and hope to clean up a few buns while there and perhaps shake the spiders out of my old .303LE (currently owned by my BIL). Cheers Brian