Pete's right,
you can't tell what kind of frame it is by the serial number, but H&R can, as for fitting accessory rifle barrels to it, even if it was an SB2, which it isn't, they will only fit 357mag, 44mag and standard shotgun barrels to it because it was made in 1998, one of the outsourced years, see the built year info in the FAQs.
Tim