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I don't think it's a Warner Swazey, especially marked USMC, but I could be wrong.
The one below is for a Springfield 1903, but 500 were bought buy the Canadian government to mount on the Ross during WW1. Some even ended up mounted on P14's at the start of WWII
BTW, this one belongs to ~Angel~, not me. She has two of them and is thinking about mounting one on a 1903 as a shooter.
Model 1913 Warner & Swasey Telescopic Musket Sights
c/w RIA M1908 Pattern Leather Carrying Case