The 10ga SB2 frame isn't marked SB2, just like all of H&R's frames are unmarked, the only way to tell an SB2 rifle frame from the SB2 shotgun frame is the firing pin and hole is bigger and the dimension of the inside of the frame is .100" wider for the 1.2XX" chamber swell of the 10ga shotgun or12ga slug barrels.
Tim