I always find it helpful to see what other companies say about installing their GPS systems. Here’s a document that describes installing an AutoFarm system on a GreenStar-ready 20 series tractor, which is likely similar to your 7930. Page 3 of this document shows accessing the steering wheel encoder harness. It’s two 4-pin connectors. Not sure why they use two different connectors. Redundancy maybe? Doesn’t say what kind of encoder it uses. Most of the cheap ones you’d use with Arduino are just 3 pins, not 4. Anyway, yes it looks like your picture there so I think you’ve found it.
Autofarm works with the Deere steering valve, so it must be something fairly understandable, likely not using CAN bus at this point. If you find your steering valve on your tractor you can identify CAN pretty easily because it will involve a twisted pair of wires .