Pic posting help can be found in the FAQs and it's stickied in all classified forums as well. As Dave said, the 7.62x39 was factory available at one time but was discontinued in 2006 due to too many owners complaining of misfiring milsurp ammo, the hard recessed primers just don't work in a factory Handi, some owners made work around modifications, but it was doomed as far as H&R was concerned. It's a shame because sporting ammo works fine as well as handloaded cast bullets and it's a real popular chambering, both as it is and as a rechamber to .303 brit or 7.62x54R, just take a look at the want to buy classifieds.
You can confirm if your .223 is on an SB2 frame by comparing it with the pics in the FAQs, SB1s look different and are made difference. You can also call H&R and ask what barrels can be fitted, it may not be eligible for many rifle barrels depending on when it was made even if it's an SB2, that's shown in the built year info in the FAQs. Sounds like someone added an aftermarket swivel stud if it's on the barrel. H&Rs have been made since the late 1800's, modern H&Rs since 1987.
Tim