Ross M1910 Pump Action conversion

[Sunray]

"...the unique craftsmanship..." I'll say. Really well done too. I'm guessing it was done for use out of a blind. Long eye relief scope with a very short butt stock would work well when sitting. Might have been for a wheel chair bound shooter/hunter.
There was a 6.5 x 57 Mauser. And a 6.3 X 53R R/K FINNISH. Be easier to convert/rebarrel one rimmed cartridge to another. Although there was a rimmed 6.5 x 57 too. Listed here under European Sporting Rifle. Warning: This site can keep you busy for hours. snicker.
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[Surpmil]

9.3x57 I would guess.

[Promo]

There is no need to guess since it clearly is marked and papered as 6.3 x 57. Thanks for the link, might indeed be a hybrid of different cartridges.