I have also aquired a couple of HP printers with the very same sensors.
One (Q9864) is used for the striped strip running behind the print-head

for position sensing, while the other (Q9845) is used in a similar
fasion
for a similarly encoded disk to gauge the paper position by the
rotation
of the rollers. I intend to use them for positioning in a CAD system,
and have also been looking for datasheets without success.

I have decided that I will figure them out datasheet or not, as they're

too useful looking to be sat around doing nothing! I'll happily tell
you
what I learn if your still looking for information when I've done some
fiddling about with the oscilloscope. So far I've only had a visual
inspection of the components and contemplated their use. I'm fairly
sure of the power connections from this first investigation, but can
only
guess at the function of the other two pins! I'd like to think one
pulsed
for left movement and the other for right... but that may be too much
to
hope! Perhaps it's more along the lines of the sensor system in a
mouse, where extra circuitry is required to decode direction from the
order of the beam interruption.

I have to show my ignorance and admit I have no idea what a
'quadrature' direction decoder is :) But I assume it's effectively a
device
that provides ready-decoded left and right outputs. Or perhaps it has
some bearing on sensitivity??

so far what I have is:

          _________
          |              |                   1: -ve
          |              |                   2: ?
          |              |                   3: +ve
 ----------              ---------          4: ?
|___________________|
            |   |   |   |
           1  2  3  4

Not sure of the voltage, but I'd guess 5v *crosses fingers*

Please let me know if you've got more information than this!