Encoder resolution?

Hi Group,
I just found 9 Sanyo Denki Step-Syn stepping motors - Type 103-820-2,
4.5V, 1.4A, 2 deg/step, with encoders attached. The encoder disk has
2 rows of slots, 45 slots each. Do I count 45, or 90 as the
resolution of this encoder? There are 8 wires total going to the
reader head. Can anybody out there provide info about this encoder.
Are higher count disks avaiable? The motor has IBM p/n 2526734 on
the back. Has anybody seen these before? I think it used to be a
tape drive.

Thank you,
Allan