In addition to 8mm-06, it could be an 8x57 or 8x60 or one of the rimmed versions. Looking at the bolt face and trying some empty cartridge cases for fit under the extractor should tell you a lot for not much effort.

In any event, it is an 1888 Commission action or some commercial or custom shop variant. An early smokeless design and probably 1890s materials. Very high pressure loads can be encountered for all of the above cartridges.

What do you intend to do with it?