That hole is for the insertion of a early short bend gas tube.
Colt toyed with this hole in the 1960s are there are a number of Colt rifles/barrels floating around out there that have this feature.
"Not every thing on Earth requires an aftermarket upgrade." demigod/markm
Rare stuff reguardless ( as far as Retro goes ) proceed taking that into consideration of type of build ...
I have the same type of FSB without front hole on a C MP C barrel so chrome chamber only, wondering if that would be correct for that barrel?
Nevermind, did some searching and looks like the chrome chambers were late 60's so 603 FSB is not correct.
Last edited by mack7.62; 02-12-18 at 14:54.
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