I have a Springfield M1 Garand which appears to have all Springfield parts except for the barrel. the only markings on the barrel are: SPA SPT PN PA. These markings are where the usual barrel markings are stamped on Garand barrels. The serial # of the rifle: 28253xx. I used my CMP throat errosion tool and the barrel measures less than zero...if thats possible. The muzzle measures zero or less also. Any idea what type of barrel this is? PAT.