Barrels have usually been marked SB2, but the frames aren't marked except for the new SB1 rifle frames which are marked that they can only be fitted with the barrel that came on it, or something to that affect as stated on the H&R website. HX= 2007, so there's a good chance it has one of the Marlin made barrels that has the tighter bore, but no guarantee since they started making the barrels in early 2007, they didn't offer the SB1 44mag until 2008. That's a bit high, see if you can get em down to the market price which is about $250.
Tim