I've been using Logitech TrackMan Marble for a while, but I didn't want to break warranty by modding it, so got few old used ones (or rather Marble Mouses, actually). The only difference I've noticed between generations is buttons: the new ones are almost the same color as the case, and have lighter Omron D2FC-F-7N switches instead of HC DM1 1A 125VAC.
However, the old ones have opaque, not really smooth balls, as you can see in the picture. At first, I thought it was a design flaw that's been fixed with the new generation, but STFW suggested they were probably glossy out of the box. I was thinking it was some sort of damage then... well, the explanation is much simpler. The old balls are actually a tad bit smaller than the new one. They're probably worn out.
There don't seem to be replacement balls available. What to do with the old ones now? Possibly apply some finish?