It seems a unique style to me - Luth-AR .308 Rifle Buffer Tube Assembly

It is a non-adjustable buffer tube which seems intended for use with the non-adjustable Luth Buttstock. A screw in the butt end attaches to the buffer tube end. The buffer tube is threaded all the way into the lower receiver (to cover the buffer stop) and also acts as the receiver end plate. Leaving no room for an end plate with a QD attachment option. It is a simple style and seems to work very well on my AR-308