H&R made some Handy handguns many years ago. Those models are perfectly legal as a handgun if they have a rifled bore. If they have a smooth bore like the 410, 28ga and 20ga models, they have to have a 18" barrel and an overall length of 26" or more. If they have a < 18" barrel they are an NFA device defined as an Any Other Weapon(AOW).
If it was registered as an AOW prior because it had a < 18" smoothbore, even without the shotty barrel now, then it must be transferred as one, or struck from the registry. If it was disassembled prior to the NFA taking place(1934) and never had a short smoothbore barrel attached it is just a common pistol receiver which follows the GCA of 1968 laws.