Went to an estate sale over the weekend, only because there was a nice pile of AR-15 stuff. Bought a box of assorted parts and one intriguing item was a 2 position extension tube. I didn't measure it until I got home and it turned out to be a commercial tube with a non-angled back end and black in color. I have never seen a commercial 2 position tube and wonder about its origin (Bushmaster, DPMS, etc.)?

Any ideas?