There may be manufacturing id marks on the barrel near the barrel extension (Under the barrel nut). The gas block looks similar to the generic imported gas blocks from China.
At first glance I thought it might have been on of the barrels from the original Smith and Wesson M&P15 Sports which would be 1/8 5r rifling and made by Thompson Center. It has the profile, looks mellonited and says 1/8, but it doesn't look like it says 5r on it. Plus it also doesn't look to have the front sight base holes drilled which a S&W would have. I don't think they offered the 1/8 5r bbl on their other models. Plus this would have been around the time right before free floating an AR really took off. Man, if it were an original Sport barrel, I'd be offering you monies for it!