The classic model sebenzas also have a weaker detent compared to the 21's. On the 21 the detent hole goes through the blade while on the classic its more like a dimple.
Interesting. I've actually been considering selling my Classic and buying a newer model. The "Classic" in big letters has always bugged me. The knife is a well-known classic design; splashing it across the scales comes off as a bit self-conscious to me. But then I'm not sure I'd like the "Idaho made" much better . . .