I have a 1918 Winchester trench gun. Barrel is 20 1/8", serial number 679XXX, proof marks on barrel, no heat shield but screw notches are on barrel, US marked top right bolt, small oval with W. The repro heat shields are .862. My barrel OD at muzzle is .873, ID is .721, barrel wall thickness is .074. Does anyone here know if these are correct and if not any idea why