Almost certainly his conclusions were based on results with the ammunition he had on hand at the time. He also reported the 10.75x68 was inadequate on dangerous game. Again because the 350 gr ammo was too fragile. With some modern makers making good 350 gr DG projectiles that is no longer a problem.

BTW Harry Manners also disliked the 10.75x68 and sold his to buy a (couple of) .375 H&Hs in Winchester 70s.

Remember before the days of hand loading the perspective was different. If the only available ammo for a calibre was all crap or inadequate, the rifle became so as well. Nowadays this appears to be an absurd way of judging a rifle. Because if we don't like the ammo available, if we shoot it at all, we just choose a different bullet to reload in it ourselves. Not so in the first half of the 20th Century.

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I had a chance to buy a Mannlicher Schoenauer in 9.5x57, and another in 6.5x54. I had never heard of the 9.5mm round then, but the guy owning the rifle used it on fallow in Tassie and Sambar in Vic. Personally I think it would make a great brush or driven medium game cartridge but would prefer something more on dangerous game. I didn't buy the rifles becuase 1. $$$, and 2. they were half stock rifles not MS carbines. Was very tempted though.