Interesting.
"FLAT WRONG"
Note that I said the correct bullet weight "SHOULD" be stamped on the barrel flats.

400, there are some contradictions in your post which you can maybe clear up.

If as you say a gun has the Cordite 43 310 Max proof marks then this must mean that it has been PROOFED for the 400-350 cartridge which is a 16 ton cartridge.
Then you say if the SAME RIFLE also has 350 No2 on top of the barrels it can shoot a load which gives similar and sometimes better ballistics (and loaded hotter) than a cartridge which is listed at 17.5 tons, the 350 Rigby Magnum!

Add to this the fact that the dimensions I have read for the 350 Rigby Magnum show it is a bigger case by a fair amount than the 350 No2.
If those dimensions are correct this would mean that for the 350 No2 to produce similar or better velocity and be as you say loaded hotter with the 225 gn bullet than the 350 Rigby magnum, the 350 No2 would have to be producing pressures CONSIDERABLY above 17.5 tons due to its lesser capacity.
And you say this is ok in an old gun proofed at 16 tons?
I wonder what pressure your mates rifle was producing to get 2660 fps in his 350 No2.

What proof is there that these two cartridges were ever actually rimless/flanged pairs?

Anyone got true case dimensions for the rimless 350 Rigby Magnum?

Bill,
The velocity I quoted for the 400-350 was from Nobel, Standard Ballistics for Kynoch Cartridges 1925.
Apologies for any errors.