Alternatively, you could very well take the corresponding Ak contact, put a logic inverter, and send the inverted recreated Rk signal to the circuit.
(perhaps add a litttle R/C to avoid recreating bounce)
the crazy encoder scheme was probably used because the software was to slow to reliably decode a standard A/B encoder.
With this scheme, if you miss steps, it just slows down, and does not move around randomly.
If you replace the encoder, please take apart the old one, it probably has some nice small german mechanical clutch magic to generate a separate signal per direction.
Company still exists, makes encoders, but does not seem to make this crazy modell any more :
https://ebe.de/en/hmi.html