The board has 2 sides. Let the side with the sensor call the first:

Sensor in the centre, and about 20 resistors & capacitors. 
Apart from that we've got about 5 small (up to 6 legs (?)) 
transistors. Sadly no interesting numbers
(not interesting means things like R12 or C17 locating proper 
res. or cap.). 
The other side is a little bit better desribed.
In the upper left corner we find a logitech logo and name. 
Then going right, close to the edge we've got:
201757-000 REV.A0
QC ELCH 10/07/2003

Later going down, about 5mm over the lower edge
we may read quite big: 493T.

And agaig going down, just on the lower edge, in the 
center of it (left to right) we've got:
CHI SHENG  33VO

Then, the only described thing on this side is something 
measuring about 10mm x 4mm with insription on it:
<T> 6.00L3
This <T> is really an outlite T within a diamond (rhomb ?), 
but it shouldn't matter :>
It is shining (as it's made of metal). I guess it has sth. 
to do with clocking device, but it's only my first thought; 
nothing beyond it.

As we should expect we've got here another 15 cap. & resist.
And 2 transistors I guess, although they are desribed as U5 & U6.
This side has also one 2 pin connector for the button,
one 5 for the usb cable. 
BTW on wire of the usb cable comes direcly into the board.


It may also be useful to include thing I've got written on 
the label attached to the cable nearby the connector.
M/N: V-UAL9     Rating: 5V, 100mA
P/N: 861154-0000
S/N: LZA40903286
Made in China...

So just to sum up, there are no nubers/letters on the sensor 
per se, as well as in its vicinity.