On closer inspection of that manual, it appears that only the M54 may have had a remote (RM-D597).
The M54 also had variable line out and headphone out.
The M43 appears to have been supplied as part of systems, so it's possible the IR control was performed by another component in the system, and communicated to the CDP via some bespoke interface.
I'm sure further inspection of the service manual will provide the answers.