I haven't heard of the cartridge before but found that that Holt's listed a 9.5x57R MS model 1910 rifle in their Dec 2011 auction:

https://auctions.holtsauctioneers.com/asp/fullcatalogue.asp?salelot=A1058+++1115+&refno=+++51832

Maybe they got it wrong or maybe it was one of those cartridges that was developed and then disappeared just as quickly. 9.5x57 MS rimless is a known cartridge, and Europeans often made rimmed version of rimless cartridges to suit popular break open actions. Maybe that's the case here. I look forward to better informed details from the many experts on this forum.