Regarding the 9.5MS vs. the Westley 375 Rimless, I wonder if the 375 Rimless charge of 43grs of Cordite could be a match to the European load of 45grs of flake powder?
My Mauser rifle is proofed at 43grs. Cordite, 270gr bullet, which is the same as the MS British proof marks just as Rothhammer1 notes above.

Also, I've read some thought that Kynoch loaded the MS load a little faster to get the imported MS rifles to shoot to the sights. That could be possible I guess as Kynoch wouldn't want the many English buyers of these rifles to be dissatisfied with the Kynoch ammunition.