I have a Stevens 16 Crack shot that I found a breach block for. I later located a second breach block that kind of worked in the rifle but it's face size was a bit smaller and didn't like working with the curved section of the newer version hammer. Was this difference due to there being an earlier model (with thumb safety). I believe that the smaller block might be an early version and work OK with the early hammer - which I don't have yet. Does anyone know about a possible reason for the size differences?