Looking for a 604 upper. Finish is not important as it will be redone in current Colt black to build a "modern" 604 carbine.

You should try the EE - the WTB threads. Please read the CoC.

You should try the EE - the WTB threads. Please read the CoC.

This.  Deadset on 604 or would a SP-1 with bushing suffice.  I have neither to offer but SP-1 does offer early square forge appearance and may be easier to find / a little less expensive. Are you going to have re-anodized or use alternative finish?  If re-finishing something fairly scratched the price should be better for beater condition and you're not effecting something with possible value down the road based on as purchased condition.  Always better for future generations to save the pristine originals.

Once it's yours it really doesn't matter anyway as long as you're happy with it.  Sounds like a great plan.  Kinda leaning toward picking up a couple in near future myself.  Someone may hit you up with a project beater.  Gotta start somewhere.  Try searching GB under Colt AR-15 also.  Usually a couple but kinda pricey at times.  I haven't looked this week.

I might be able to help you......IM Sent