>>9105282They were included as extra parts in some Nuva sets. Bonus gear pieces were also included so you could change their gimmick from swinging either both arms or just one. The balls were probably just there for the Kaita but I've seen some people put them on their back. This is a picture of a freshly opened Lewa Nuva from the BZP archives, and you can just make out the ball pieces.
Unrelated, but I also found out that some of BZP's old links now lead to porn sites.