Gas Block With Threaded Front End?

[stevegun 1]

Can anyone identify the gas block pictured here? It came on a 6.5 upper that was test fired only and was traded to me. It appears to be threaded 1 1/16 x 28 TPI, although the major diameter is is 1.043" which is less than spec. What was the threaded portion used for? Any thoughts are welcomed. Sorry for the poor focus on the photos.

[bwaites]

That's AA's gas block.

Hopefully Bill Alexander will be along to explain the threads! Its a good story.

[Variable]

A guess--- Reflex mounted can kinda like KAC???

[stevegun 1]

Thanks Bill, it seems like it was an old school design and would accept a "cap/plug" to seal off the front end of a free float tube. Variable, I do not think it's for the type of unit you mentioned, not in this application at least.

[Fireside]

Did anyone figure out what the threaded gas block is for? And what device will screw on to it?
I just got one with my 20" AA Grendel upper and manticore hand guard.)