I don't know. I have just heard from sources I trust that at some time a change was made to the metallurgy of the splines. Supposedly, it's the later Monolever and the Paralever drives that got them. If that is the case, it may have started around 1989 when the first Paralevers came out.
Then again, I have seen photos of drive shafts and final drives from Monolever bikes as late as '94 that had pretty torn up splines. As for the Paralevers, my impression is that on those the problem is exploding u-joints.
I have bought five K bikes with less than 54,000 miles and one with 108,000 miles and all had good splines. None of them had any record of replacement drive shafts.
