Calibre Options for Ross M10 factory sporter?

[Surpmil]

Overall it appears to be an M10 .280, even to the barrel profile, but clearly those markings on the barrel don't fit that hypothesis.

Is there a serial number on the barrel just above the stock line and just ahead of the reciever (offhand side)?

At a guess I'd say you may have a .280 that was later converted to .303 using a re-profiled and shortened Mk.III .303 barrel.

Trouble is, you don't have a .303 magazine there - or at least not the factory one - what does the feed ramp look like?

What type of butt plate does it have?

The stock is either a copy of an M10 stock, or someone has crudely recut the chequering. I expect it is the real thing, but it looks like a Mk.III butt plate might be fitted?

It has obviously been together for a long time, and it is known that a few Quebec City gunsmiths assembled Rosses from leftover parts into the 1930s even, so that is another option.