The S stamp at the rear of the handguard, I found it in "Springfield 1903 Service Rifle. Production and Alteration" by C.S. Ferris and John Beardicon. The handguard was reduced in length by 0.2 inches and the letter S was stamped near the rear sight swell to indicate the handguard had been shortened from the 1905 handguard. I have a handguard with the letter S stamp too, but my handguard is also with the two handguard clips but does have the rear sight cut