the 401 has a 'cam stack' - meaning that the cams are more or less built it - you choose the stitch you want by 2 different levers... it also uses other discs
so if you come across a 401 but it's missing the extra discs grab it anyway - you can still use a blue bazillion stitches while you look for more discs...
I found a really neat blog:
http://blog.sew-classic.com/2008/10/...ne-review.aspx