All of the answers to your questions are in the FAQs, the first letter N or H denotes who's name is on it, N= NEF, H= H&R, both made by the same company, just depends on which model it was originally on. The second letter denotes the year of mfr. The only way to know for sure if it's an SB1 shotgun frame or and SB2 rifle frame is to call H&R and ask, they can tell you by the serial number. There are also pics of both types of frames in the FAQs, there are some differences that are obvious.
Tim