Yes, the Huntsman frame is a SB1. The reason it doesn't fit now is it needs to be properly fit to the frame. There is no "legality" issues as the origional purchaser should have filled out proper paperwork as if it was a firearm and not a black powder rifle. (Yello form 4473)
FYI, The SIDEKICK is a black powder only frame and cannot (easily) be made to fir any center-fire barrel. The Sidekick is NOT considered a firearm. It is a Black powder rifle.
CW