As far as caliber, have a gunsmith do a chamber cast to see exactly what you have, anything else is just speculation. It may be a wildcat so you will never know, I doubt it is Britishicon so probably not a standard .303.

Very interesting. As Calif-Steve says, the rifle has had a Gew 98 Lange Vizier rear sight added to replace the original Gew88 rear sight. And it has had the magazine blocked to disallow the use of the magazine meaning it is single shot only.

I vote for a post military, one off, target rifle. And definitely done in Europe.