The 10ga uses an SB2 shotgun frame, not the same as an SB2 rifle frame, only 10ga and 12ga Ultra Slug Hunter barrels will fit the SB2 shotgun frame properly, they're 1.2" in diameter at the chamber swell compared to 1.1" for all other barrels, the SB2 shotgun frame has a larger opening to accomodate the larger barrel. The forend spacer is the same for all modern H&R/NEF single shots, plastic, not metal unless someone has replaced it aftermarket.
Tim