Dimensions are important. But there are several ways to use any encoder. Adapter rings, couplings etcetera.
Next thing is number of channels. You can always find out if there are two, three, four and so on wires connected to it. Even if your encoder as such is next to non-existant, you can look at the other end of the cable and see how many are used.
Then you need to know voltage. Just measure it at the controller's end. Expect anything from 5 V to 35 V.
Last thing to find out is number of pulses per revolution. Connect an oscilloskope and look at the pulses. Simple calculation (measure shaft speed also) will tell you if you have 500 PPR (common twenty - thirty years ago), 600, 1000, 1024 or some other number.
Also, if you can identify the machinery it is attached to, that may help.
Out of curiosity: What kind of machinery is this? And, where in the World is it located?
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...