Civilian Colt AR15's had no provisions for the push/pull front pivot pins untill the late 80's. I was wondering if the eariest Armalite/Colt AR15 select fire or M16's had the flat sided receiver like the early SP-1 Sporter Colt's with the double machine takedown screw or did they always come with a raised fence to accommodate the takedown pin, detent and spring?  Thanks.

None of the above.  Colt 601 rifles came with a slab-sided receiver, but they had a straight pull, non-captive pivot pin.  The fact that this pin could easily be lost while the rifle was disassembled is why the DoD insisted a captive pin be developed before the rifles could be officially purchased.

Did it have a detent of some sort built into the pin and a head on one end?  I have only been able to find good pictures of the left side of a 601 but if I remember right, the one right side photo I did find didn't seem to show a head on the front pivot pin.

Yes the detent ball and spring were part of the pin it's self. The 01 and all later AF M16 / A2 Upper recvr's have a counterbore on the left side of the pivot pin hole for it.

Pic of AF A2 Conversion upper with Mod 01 Pivot pin detent counterbore.



The whole pin was easy to lose and the ball and spring had the tendency to come out of the pin making the pin pretty much useless and even easier to lose...