Looks like it's the 16" A2 flattop, with the Vitor flash hider. Brand new, never firs with manual. Still looking for $700 shipped, that's about $100 below their site. Alexander Arms site. I'lll have to check on what I made, so if you are seeing it cheaper and want it, just send me an email and I'll consider it.
Free shipping from me.
They told me it is a 3/4x28 pitch. I don't know how to determine this, but that is what they said this model typically is. This is the 16" TACT model. Currently not in production. It's just an upper, complete with magazine and manual, never fired. Ready for an upper. It's on Gunbroker now, so I have to consider it SOLD. Thanks for looking.