It could have been marketed as a barrel tensioner.
Years ago I bought a similar device for a Century 91 I had at the time. Made it look like a heavy barrel.
Born in the US of parts made in Emmingen, Baden-Württemberg.
Devastation to your doorstep in 30 minutes or less...Minuteman III LGM30G.
Awesome it worked out for you
That's great news! I had a smaller, similar device on my SAR-3/8 (Overstamp) and was able to remove it to reveal the original threads intact underneath.