What I found out:
968.51 = Black, 1.8 m cable.
968.52 = Black. 2.8 m cable
968.55 = purple, 1.8 m cable
The other ???? I can't see the real difference between the 968.50 and 968.51, except that the 968.50 is a bit cheaper. I assume that it does not have something that the 968.51 has.
Anyway, we (or rather my wife :-)) have been using the 968.51 almost every day for about 3 years. My wife is very satisfied with it.