I think you're indicating the "castle nut" that's on all AR15/M16 pattern collapsible stock tubes. It doesn't have a set screw, it has little holes so you can use a stock wrench to tighten it down. It holds a metal plate on the back of the receiver to hold in the rear take down detent spring.
They changed some not long ago - you want the pre-M4 type.
As to the stock itself - it looks like they just welded (or something) an oval plate to the back of a regular carbine buffer tube. Note the "square" part along the bottom of the tube. If they used functioning ARs with blanks it makes sense they would have kept this part - you need that tube for the bolt carrier to recoil into as the rifle cycles.
Anyhow - just look for an older style "CAR" buttstock assembly. It'll run you 25-35 dollars prolly, and comes with the tube, the back plate, the retaining nut, and some assorted other stuff you can just trash. You DO NOT want a "postban" type, which has the stock part permanently affixed to the tube. On the "preban" types you can just pull the stock off the tube easy as pie. Look for the older smooth-sided type, not the modern "M4" type. The newer one has a slightly different nut and (I think) is a little longer. Might have an angled back to, can't recall. I don't *think* so though.
edit - ah - like this one:
http://retroblackrifle.com/ModGde/CrbGde/CmpGdeCrb.html
(2nd or 3rd generation)
and here's one for sale. It has the newer castle nut (also called a receiver extension nut) - the older ones just had holes, not "castle" looking cutouts on one side.
http://www.galatiinternational.com/...Screen=PROD&Store_Code=GIO&Product_Code=MD100
edit again - looking at LizardJedi's picture of the rifles on the table, I'm wondering if the picture you're pointing to Sluis is a firing "hero" version. It pretty clearly uses a real buffer tube under all that getup, and though it's hard to see at that size and angle, I *think* there may be room for a gas tube under that upper "barrel" part. If so, even without a visible magazine it could have been modified to fire say 5-7 shots from a modified flush fit magazine without reloading. I see the charging handle is missing, but that could have been removed later.
The rifles on the table though don't use the buffer tube - those stocks are too narrow in diameter to accept all the recoiling greeblies that spring around on the inside on the real rifles.
So.... maybe a blank firing hero? Or perhaps fan made after all? I couldn't say.