I've always loved A2 styled guns. The flattop just doesn't do it for me. To that end I stumbled across an excellent condition Colt R6551 blue label with original box with features I'd never seen before.

It's a large hole front, small FCG pins, no sear block, A2 lower, A2 upper, bayo lug, gov't profile barrel, roll marked SPORTER TARGET. It's IDENTICAL to an M16A2 aside from the front pivot and fun switch.

All the guns I've seen of this model lack something. If they have no sear block they're usually a slab side lower, or are missing the lug, or are sear blocked AND missing the lug, etc. Ive even seen one a good couple thousand off (higher than) my serial number with a slab side lower so obviously this model number was a transitional period for Colt.

I paid $1200 for the gun and I'm very happy with it, could continually hit a 6" steel gong at 100 yards with irons standing offhand. I'll try to get some pics up soon.

It's hard to say how rare they are. You definitely don't see them very often, but they are out there. I have a blue label R6520 with the exact same features; large front pin, small FCG pins, full mag fenced lower, and no sear block. It is one of my favorites.

I'm not sure I've ever seen a large pivot pin in conjunction with a full fence. Can someone post a pic?

The Colt MT6700 has exactly what your saying you've never seen.

ETA: My first AR was a late 80's Colt Sporter A2 with a full fence small pin lower, 20" gov. profile CMP 556Nato 1/7 barrel, A2 FH, A2 upper with a notched big head forward assist. The gun came from the factory like that with a black sling, 2 mags, and a greenish/blue owners manual. I got it for Xmas one year, and I kept it in the box under my bed with a cable lock through the port and the takedown pin pushed out with the receivers cracked to get the cable lock to close. I remember trying to find a longer one that would go through the magwell and out the port and back into itself but no dice. Heh.. I was 13 years old, and it was my favorite thing in the world at the time.

I tried googling for pictures but didn't find anything which showed the large pin with fence. My ban-era LE6700 (made during the same era as the neutered MT6700) had the small, two-piece screw, but I had it drilled out by Specialized Armament so I could use a normal pivot pin. Here's what it looked like when I bought it.







 
 
 

I don't have a pic of the other side, but here is a 6601 rifle (replaced the barrel with an A2)that I had a few years back.

Full mag fence, large pivot pin. You can see the aftermarket pushpin, as well as the shape of the lower.