True about the Schuler - pretty close to a .450

I think we mistakenly believe that the 450 "ban" was an all out ban on .450 ammunition but it really was a restriction of import.
Sportsmen could still bring .450 ammunition into India and British Africa but were limited to something like 200rds per year. There was still 450 ammo being loaded at the time.
Still, one would think that with the advent of the 470, 475s and 500/465 as a measure to circumvent the restriction (and sell a lot of new rifles as well) there would at least have been some sort of production rimless 450 class cartridge to come out of England.

Perhaps part of the allure of the sporting single shot rifle is that they mostly predated the advent of the repeating rifle, namely the Mauser '98.
Good single shots, even Martinis, just have a different era feel to them while there isn't much between a prewar Mauser sporter and one made to that pattern today.