It never really slipped, if it was fully engaged. That was kind of a trick though, if I didn't let the clutch out fast enough it could slip, sometimes, but clutching in and out quickly fixed the issue every time. It would hold all the way through the rpm range if it was holding normally and slip as early as 4k if it was going to do so.
It was also very difficult to get any slip to get the bike moving as you normally would, I usually had to add lots of rpm and then let it out quick. The clutch would slip just enough to get it rolling and then grab, but it was a real pain. If I had to deal with more stop and go traffic I would of done this a long time ago.
Do you remember if all of the after-market ones were longer than stock? I wonder if someone put a set for the wrong bike or year in.
So, you just went back to the old stock springs and flipped the washer on the bolts? I might have to look at mine again, I can't exactly picture the stepped washer you're referring to.