SB1 is the shotgun / muzzle loader frame. SB2 frames are the rifle frames.
Here's the official word from the H&R site.
* Additional barrels may be added to firearms of our manufacture since 1987 only, with a serial number that begins with N or H. We do not install additional barrels on any other manufacturer’s firearms.
* Additional barrels are available as additional barrels only. We will not exchange barrels for existing barrels on shotguns and rifles, as it will change the model.
* Rifle frames may be fitted with additional rifle, shotgun, and muzzleloader barrels.
* Shotgun frames may be fitted with additional shotgun and muzzleloader barrels only.
* We do not sell additional barrels outright. All barrels must be factory fitted.
* We do not sell barrel blanks or do custom calibers.
* In order to obtain an additional barrel(s), the stock, forend and frame (receiver) must be sent to us so they may be properly hand-fitted to the barrel, to be sure they are correctly head-spaced and proof-tested.