Now how you supposed to tell which is an SB2 and which is an SB1 if you have only one or the other without a barrel, I do not know.
This post is for everyone, not just you Fred.
There are three things that stand out on a SB-2 rifle frame to be able to tell it apart from a SB-1 shotgun frame. Wish I had my new digital camera, maybe someone could take a pic of the differences and post them in the stickies. Quick? I know you don't have a shortage of frames. :grin:
The front of the SB-2 frame where the forend hinges has a lip machined around the radius to where the spacer only makes contact with the outer most edge. The SB-1 frame will have a uniform radius all the way accross. You will need to remove the forend to see this
The inside of the SB-2 frame is machined smooth while the SB-1 frame will show the casting marks from the mould. You will need to remove barrel from frame to see this.
The firing pin on the SB-2 frame is much smaller in diameter than the SB-1 frames shotgun sized firing pin. I believe the smaller rifle firing pin and firing pin hole is for keeping the high pressure rifle's primers from flowing back into this hole and therefore locking up the breech.
There may be other differences, but those are the ones that stand out to me. The older rifle frames may be different, my SB-2 example is about 5 years old and it is a Ultra Varmint. I also have several SB-1 examples and they are consistent.
Hopefully someone can post some pics of a side by side comparison of the two frames.