I performed some searches on this subject and found zero topics...so here goes...

I recently rescued my AR from the back of my gun safe.  Too busy shooting trap/skeet and action pistol.  Anyway, I have been looking at a possible second upper, but I am confused on the large pin / small pin discussions I read in these posts.  

I have a Colt ser# SP 177XXX.  I purchased it used over 10 years ago and I am not sure how old it was at that time.  It has no forward assist, windage only adjustable rear sight, and came with triangular hand guards. It also had a screw type front pivot pin.  I replaced the screw pivot pin shortly after buying it with a push pin that’s only drawback is it completely separates from the rifle on disassembly unlike the rear pin which is held in place.  It has never came out accidentally.  The pin is approx 5/16".

Is this the large pin I keep reading about?  Can I assume only uppers with this size pin hole can be used on this lower?

Colt large pin, you can use an adapter to use small hole uppers, but IMHO, they are PIA.

Not only are they a PIA, but some of them will rotate loose.  I have one that utilizes a 90deg flip lever instead of a second screw head.

However, I just couldn't take it anymore and built a large hole upper to the configuration I wanted.  It was all gray, to match my early Colt to boot!  It was based on a Colt 6700 large hole flat top receiver I score on the EE.  Now I use a large push pin.

Thanks for the info.  I'm hoping to get a 9mm upper in the future:)