To suggest that W.R. used a standard action simply to make the gun cheaper to manufacture is ridiculous.
The fact is that the magnum actions weren't very much more expensive than a standard action. Also, think about whether any company other than Rigby could even source a magnum action from Mauser - well, they couldn't.
That is why original 404 Jefferys (1909), .333 Jefferys (1908), 318 Westleys (1910), and 425 Westleys (1909) were ALL built on standard actions - they had no option at the time! And I haven't even mentioned the .500 Schuler, which btw, was the most powerful cartidge in the world at the time!

Anyone with access to both a std. and mag. action should stick their finger in the front ring from the magazine side and feel the difference in metal behind the bolt lug swathes - not much difference is there?
Also, IMO, the cutout on the rear of the front ring is only needed if you plan on loading from a stripper clip - and as mentioned above, only if you have a very long bullet.