What you have is an Colt SP1 upper assembly. Not a 604. The 604 uppers did not have the .315 hole for the pivot pin. Only the SP1 uppers did. 603 uppers have forward assist. I have the exact same upper assembly on my 1977 SP1. The C MP CHROME BORE barrels were used starting in 1974. M16A1s used the same barrel up until production ceased (around 1982). The C M forge code is for "Colt / Martin Marietta".
"A Bad Day At The Range Is Better Than A Great Day Working"
USMC Force Recon 1978-1984
US Air Force Res. 1995-2004 (Air Transportation)
M16/AR15 shooter since 1978, gun collector and AR builder since 2004
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