Islandarms, it's a 30-06.
The advice being given here is fine if you don't have any indicators.
Your rifle was rebarreled, and as mentioned, either long or short chambered when ordered, intended to be finished by the installer, with a finishing reamer to establish headspace.
The stamp indicates it was "long chambered, which means the installer can trim the tenon to establish proper headspace.
Usually, the rifle's muzzle is recrowned to remove the stampings, which could effect accuracy by deforming the muzzle and the few thousandths needed to clean it up would fix any issues, if there are any.
If you have a 6.5mm bullet available, it should fit easily into the bore at the muzzle, as will a 7mm or .277 diameter bullet. A 30 cal bullet will fit part way up the ogive of the bullet.
I've seen and used these barrels before, when the job was a one off, and I didn't want to put out money for a reamer.