John
I agree with all you have said in your latest post on the subject, with differant arguments from differant perspectives with both being more or less right.You are also correct about issues with early bullet softness but you have not added that the other issue with the 10.75x68 was the poorer sectional density than many of the large medium bores of similar calibre e.g. .404, .416 and .425. The factory loaded 347gr bullet was fat and relatively short and even in FMJ apparently did not give reliable straight line penetration.

When comparing the 10.75x68 and the 10.75x73 (.404J) the 400gr bullet is seated and crimped to the correct depth using the full length of the case neck without protruding into the powder space. To use the same 400gr bullet crimped in a 10.75x68, the cartridge overall length would most likely be too long to fit the standard magazine and if the bullet was seated to fit the magazine, the powder space would be compromised and the 400gr bullet could not be crimped.

In saying this I have a number of original 347gr RWS SP bullets which do not have a crimping groove or cannelure so I guess these may have been designed to be held in the case neck with some sort of shellac type lacquer. Have never come across a loaded round with these bullets which are steel jacketed soft points with a flat meplat. A loaded Norma round I have is a round nose with a big deep hollow point. I would think that this one would only be useful on very small light bone plains game. I certainly would not trust it on anything else.