The RSM that he is referring to is the actual rifle, not cartridge. Ruger Safari Magnum.
I have one of the Ruger Express Rifles in 30-06. I bought it used, and have not gotten around to firing it. Integral Quarter Rib is the term that you looking for regarding the sight base.
The actions are Mark II's as noted in the prior posted info. Some are of the non-control round feed, the first version of the Mark II action. My 30-06 action is one of the first versions. Pretty much a scaled down RSM. I like the looks of mine. The bluing on the actions can take on a slight purplish hue with age. At least mine has. As a Model 77 round top in 7x57 has also.
@ldmay375 has accurately described the Ruger premium rifle lineup.
Bill Ruger built a proper African hunting rifle in 2 action lengths, with integral quarter ribs, (7 hours of machining just to make a single barrel), 3 leaf express sights, barrel band sling attachment (in 2 different layouts).
Ruger never made a decent profit on these, but Bill Ruger wanted to build and sell them. God Bless him.
When he died, the new management phased these out, and developed the African lineup, as a less expensive alternative. Nice rifles, but not an Express.
After owning a couple RSEX (Express, standard-length actions, .30-06- and .338-Win Mag) and RSM (Magnum length actions, .375 H&H, .416 Rigby, .458 Lott), I have kept the 30-06. Should have never sold the .458 Lott, as it balanced perfectly in the Magnum action.
The .338 Win Mag RSEX was a beautiful rifle (excellent wood, fit and finish), I sold it as I just could not shoot it well due to the recoil impulse, stock dimensions, fit, etc.
I would buy another, if available, now that I've have found a gunsmith who does excellent stock fit and stock making.
If you are considering a .340 Weatherby, and can handle that recoil, then BUY the Ruger RSEX in .338 Win Mag, and don't look back.
You can always buy another Weatherby, but Ruger RSEX is a classic of the past from American, large production firearms companies. Many custom gunsmiths, building fine rifles, can build the equivalent quality, but you will pay a lot more.
The prices of the Ruger Express, and Magnums are only going up.
I kept my .30-06, RSEX as one of the last guns I would sell. It's just nice.