Please Note
The mouse you receive may or may not have 1.1 on the label.
This
is because Microsoft Intellimouse Optical 1.1 units sold via
retail channels (in full retail packaging) did not put 1.1 on the bottom
label. Only units sold via OEM channels had 1.1a on the label. The
presence or absence of 1.1a on the label only serves to differentiate
via which supply chain (retail vs oem) it was intended to be sold through. They are 100% the same, exact, identical hardware.
What matters most is that it has
a PID # of 55250, which indicates it is a 1.1 or 1.1a variant. There are two additional ways that you can tell an intellimouse optical 1.0 from a 1.1 by examining the base. The 1.1 uses a clear round plastic guide for the optical sensor, located in the center of the base, which you can make out through the semi-clear red bottom. Whereas the 1.0 uses a large square shaped guide, which can also be seen through the semi-clear red bottom. Also, the section that contains the bar code along with various numbers runs straight across on a 1.1 model. On a 1.0 model, that barcode is inside of an ellipse and the lettering above it is contoured around the ellipse.