Maybe this will help.
Got the latest catalog from All Electronics and in the back they have the following:
Wheel w/Hall-Effect Sensor
2 for 1.00
Hall efect sensor with a detacable rubber wheel. Basically two pulses per revolution! Wheel diameter is 0.53 mounted on metal bracket.
CAT # EX-202.
web site:
www.allelectronics.com
Happy Hacking!
PS they also had a bar scanner but that another topic...