I needed a Microsoft IntelliMouse that is compatible with the serial port and ran into http://www.ebay.com/itm/Microsoft-IntelliMouse-PS-2-Serial-Compatible-P-N-68874-OEM-NEW-OLD-STOCK-/332288143196?_trksid=p2349526.m2548.l4275 but noticed it was different that my existing one. It came in a sealed bag, has no yellowing, smooth shell (my other ones have a slight texture) and has a really short serial number, not the iconic product id.
I'm thinking that it may be fake.
It is listed as leftover new stock so it should seem a bit different from an old used mouse. Do you have any pictures of it?
I don't have any pictures because my camera is broken. 
So I'll use one from the ebay listing.
All markings are identical, except for the serial number.
Here's another one that's the same as my old mouse, only the product id is different
(sporting that familiar texture)
Product ID numbers will change all the time even on the same model of hardware. This is because manufacturers often have to change the vendors of products for many reasons. Some of these reasons might be as simple as cost or supply of a specific chip
I know those numbers change, but the overall format?