I recently purchased an M16 upper with no forward assist. It doesn't have the Colt markings but it has a small triangle mark turned sideways and a square with a circle inside below it. Both marks are placed towards the front of the receiver. Any ideas what brand and or era this might be? Thanks.

It's Colt.  Those are the standard Colt proof and inspection marks you've described.  If it doesn't have the "usual" "C" and a letter indicating the forge, it identifies the piece as a very early version, produced when Colt was the only outfit making ARs, and using only one forge, so there was no need to ID the piece any further.

If it's a big hole, it's off one of the first SP1s. If it's a small hole, like I've got,it's from the first Air force M16 contract.

If it has the original barrel on it, make note that it won't be chrome lined.  The barrel will either be unmarked, or else have only an "MP' somewhere

Thanks for the info. That works out pretty good for then. I just bought a lower dated 1965 so the earlier the parts the better. Although the barrell does have CMP Chrome Bore on it so thats from later 60's to 70's I imagine. Now that you mention it, the square that I thought had an O in the center it did look more like a C. That would explain why I couldn't find any reference of these markings. Thanks again.

Come to think of it, the uppper receiver has the large cuttout underneath towards the back for the bolt. Does that mean then its from an M16?

If it's got a small front pin hole, it's off an Air Force surp M16. If it's got the big front hole, it's SP1, and yes, any number of SP1s were built w/the auto sear clearance under the upper. The "experts" who tell you otherwise are wrong.  It's true that most SP1s and (I think) all SP2s didn't have the clearance, but my own SP1 came w/it, and it wasn't until I replaced the smoothside upper w/ an SP2 type w/forward assist that I ever saw an unmilled piece[:D]

Its the large hole from an SP1. The only thing that is not original is the barrell and bolt/carrier assembly. I did find one smooth side bolt for sale but is has most of the top cut away at the back. Were SP1 carriers like that or M16s?