After H&R Inc went out of business in 1986, they never made high pressure frames, just cast iron, H&R1871 made the first SB2 in 1987, actually late 1986 I think, but their cut off for accessory barrels is 1987 and later, and now only 2000 and later frames can be fitted iwth high pressure chambering. H&R history is in the H&R Misc forum sticky, frame strength is discussed in detail in the FAQs and Help sticky above.
Tim