This is a good example of "recycling" by a local gunsmith. Mausers are perhaps the easiest actions to fit another barrel onto. At one time, as late as about 1980, you could still buy 03A3 barrels as surplus from the DCM (now the CMP). Cost was less than $2.00 including shipping. The 'smith then cut off the 1903 threaded portion of the barrel, rethreaded to Mauser, and rechambered to .308. It worked like a champ, was inexpensive and shot as well as any commercially made barrel. It also worked very well for old timers like the .30-40 Krag. I've had several and this is what you have. Regards.