Which model back is it? Is it a 12 back, an A12, or one of the newest style backs? I bought a newer-style A-12 once that was in excellent cosmetic condition, but it was worn out from heavy use, and the seller refused to repair. I had it repaired once, on my own dime, but it still wouldn't work right (the spacing was always off). If you have an early 12 back, if the issue is the little delrin stop (which back in the day wasn't actually delrin, but I think hard rubber or some other plastic like bakelite), you've got yourself a paperweight. In the 12 backs, if the stop wears out and breaks, while it CAN be replaced, it rarely works well as making a new stop to the precise size is rather difficult. I don't know if/how this has been addressed in the A12 backs, but presumably the whole sub-assembly can still be replaced with factory new parts.