Another unfortunate, the balls from the Rubix Tower are too big. I got one and tried one of the balls and it was too big. That ball on the drone has to be 6mm or less in size.
6mm is too small, that'd make it the size of the tube above it. In my digital, I have the ball at 1cm. It is very close.
There are a number of variations on that toy. Likely not one of the eastern european versions, but still... ;-) I also looked at the entrapment game, which iirc doesn't have the ridge on the ball.
Does anyone reading this thread have a Revell 1/96 Saturn 5 model kit? if so, could you measure the size of the small round ball tanks used on the second and third stages? I just had a thought that maybe one of those could be the ball tank we are looking for.