luddite, I checked on comp.sys.apple2 and couldn't find the thread you mean, but actually it's pretty clear:

http://docs.info.apple.com/article.html?artnum=7522

http://docs.info.apple.com/article.html?artnum=6383

http://www.apple2.org.za/mirrors/ground.icaen.uiowa.edu/MiscInfo/Hardware/mouse.iic

It's definitely manufacturer related, but it seems to be those made outside the US, which as far as I can tell are only the Platinum mice. The same design to be used with the Mac Plus, IIe Platinum & IIc Plus. As far as I know, none of the original beige mice, NOT designed for the IIc, will work with it. So, only the cream colored IIc mouse and US built Platinum mouse will work on a IIc.

I would speculate that Apple tried to create one mouse across their product line to reduce costs in the old technology, which the US Platinum mouse did, but failed to communicate the update to their foreign manufacturers, or didn't care as the US supply is the only one that stocked IIc orders.

In any event, that's what I'm trying to determine – that what I've learned is accurate and complete.