Posted: 11/2/2015 3:05:52 AM EDT
Just a guess, but it is Model 1 Sales?
or this one:
After doing more digging, I've seen other Model 1 Sales threads that note a M1S barrel marking, similar to what your photo shows. Plus, the gas block in your photo looks like the they offer with the barrel I lined to above.
Good luck, and I hope that helps.