Looks like the one with the special pin so you couldn't match an M 16 lower to an SP upper.Logic always escaped me.
Indeed, no human being has yet lived under conditions which, considering the prevailing climates of the past, can be regarded as normal. John E Pfeiffer, The Emergence of Man
Those who can't skin, can hold a leg. Abraham Lincoln
It wasn't logic. In '63 Colt submitted a plan that included nine modifications to the service rifle. The Treasury Dept response stipulated one more change. The front pin needed to be offset.
Most original stipulations became unnecessary when ATF focus shifted to internal receiver machining (the shelf).
The M16A1/SP1 configuration is still my favorite.
Posts: 670 | Location: Dover-Foxcroft, ME | Registered: 25 May 2002