You can't go by serial number. Read the FAQs on the stickies at the top of the H&R Centerfire page. All the info is there. Mine's on a SB1, but I'll change that soon. The Huntsman doesn't need to be on a SB2, but I prefer them. If it was bought new as a Huntsman, I'd bet it is SB1. You can put the .357 and .44 mag barrels on that one and that's all, other than shotguns.
Good luck, Happy New Year
Pete