Seller said a standard M98 bolt entered and closed fine.......? Just no attempt to go farther as to install to be shootable.

Headspace should not be insurmountable because if nothing else, due to no available loaded ammunition, this is a totally handload only proposition. If close enough then fireform brass to fit the chamber and headspace.

Here's the proof marks and serial number on the action ring; will have a look under the wood when I take delivery to see if there are any other markings such as Vickers-Armstrong, etc.