I had a chance to inspect that one yesterday - from closed bolt face to muzzle brake front end is 19.5" - then actual barrel muzzle is 1.35", inside, back from that front edge of the "brake". So that barrel appears to be 18.15" from closed bolt face to crown. However, it appears that might be a home made "brake" - bore hole through the brake was circa twice the diameter of a .308" bullet (I did not measure that) - can see through the slits on the sides - look right through - so, must be cut through at more or less 90 degrees to bore centreline. An acquaintance describes that as recipe for a "loudener", not a brake. About worse found - with flashlight - appears that one "pin's hole" is drilled through into the rifling - do not see the other hole inside that rifling - might be full of crap or might not have been drilled through. That "brake's" body that is "pinned" may also be soldered to the barrel - has what appears to be bead of solder around the "brake's" rear end. We were unable to confirm whether that was threaded or not. As per the OP owner - does not do much at all to reduce felt recoil - we found several reasons why that might be so.
Auction pictures showed the safety "knob" to be round - probably metal - not the angled rectangular plastic original - with the barrelled action removed from the stock, it appears to be an original Rem 788 trigger (identical, except for larger attachment screw, to other right handed one that I have here). Is nice checkering done on wrist and forearm. "Checkered" steel butt plate with "widow's peak" was done fairly well.
Modifying 788 triggers used to be fairly common, I have done a few myself to make them adjustable (still have the drawings of how to do that here somewhere). Replacing the safety knob isn't too difficult either. Perhaps just a owner modified 788 carbine. 308 was the most common chambering for those. - dan