Colt's first AR-15 was the Model 01, a direct copy of the ArmaLite AR-15 that Colt's had purchased the rights to. Just over 13,000 of these rifles were produced, and production ended in 1963. 8,500 were delivered to the Air Force, 1,000 were used in project AGILE in Vietnam, a few were purchased for use by US Navy SEALs, and the rest were used in various US Army tests, to foreign governments, and sold commercially.
The Model 01 with the 1/14 twist became legendary for creating particularity nasty wounds in Vietnam during project AGILE.
Anyhow, finally found one. It had major rust and pitting on exposed areas, just got done parkerizing it yesterday. The dark band is Norrell's, I had tapped the gas port shut before I blasted and parked it. Took the tape off the port, then plugged it, and shot Norrell's around it.
The barrel is stamped with two W's, one under the FSB, the other just behind the handguard cap near TDC. There is also a M stamped on the exposed part of the barrel between the legs of the FSB on the right side. The barrel also has a witness mark at TDC, and the gas port hole is counter bored. Bore is six groove, 1/14, and broach cut. Barrel is turned down to a much smaller diameter then later barrels, such as used on the 602/603/604: