Edit: I just realized you are looking for the z motor. This is the main motor. Seeing the 131 reference primed my brain for the main motor.
http://maslowcommunitygarden.org/Maslow-CNC-Motor.html
There was a lot of discussion on this and how to count the gears to determine the ratio. The mfg had it wrong (131, I believe) and that info has been spread around, unfortunately. Only way to know for sure is to buy from the link.