Hi Rhodes,
Please allow me to explain what I was talking about in my post above.
First let me say that I do fully understand the difference between headspacing on the shoulder compared to headspacing on the rim.

The 400-350 Rigby is a cartridge that I do know VERY well.
I have a double rifle chambered for this cartridge, and have been reloading, shooting and hunting with the 400-350 for close to 20 years.
That double is my favourite hunting rifle.

Now the reason that I mentioned handloading is because cartridges which have very small shoulders CAN be presisely matched to the chamber/s they will be used in by careful handloading.

However, rimmed cartridges manufactured in the early 1900's were rarely 'precisely' matched to their chambers. They were made to easily fully enter chambers which were were often made to inexact dimensions.
They didn't need to fit precisely because they headspaced on the rim.
Case life meant nothing back then, as nobody was reloading their cartridges.
Reliable chambering and reliable ignition were important, and the rim provided both.

That is the reason I said that a RIMLESS VERSION of an existing RIMMED cartridge which featured a tiny gently sloping shoulder, may not have been popular back at the time. (early 1900's)

Note that around the same time Rigby's 400-350 NE was also available in bolt rifles, but Rigby chose to stay with the rimmed case rather than create a rimless one. This despite requiring major modification to the magazine of the mauser 98 which was used.

It's interesting to note that Sarge's 400-360 WR Rimmed case actually has a SMALLER shoulder than the 400/350 Rigby.

To finish off, the 400-360 WR Rimmless would most likely have enough shoulder to headspace for reliable primer ignition with accurately headspaced handloaded cartridges, but back in the 1900's there was only factory produced ammunition.