I think they are optical encoders, not mechanical. I found some info about them:
Based on model # from Ebay, model: 91Q128-43-00110
http://www.electro-nc.com/products/900index.shtml
If you click on "Ordering" on above link it explains how to read a model #, based on this mine is definitely optical encoder
( I cant post link cos its done in frames
and it will always display the same URL)
Everything is there, specs, features, wiring diag.