The TM751 is also hard coded to Unit Code1. For the House Code set on its House Code dial.
You can't turn it on and off with a power line command. Only an X10 RF remote can change its state and it is also not polite. So it can step on a power line command it does not see.
The RR501 can be set to Unit Code 1 or 9 for the House Code it is set to. It has a power line receiver in it. So both X10 RF and power line commands can change its state. It should also see any power line commands it received and politely wait to send what it received.
The PLC01 has no X10 RF receiver in it and only repeats an X10 power line command on the power line. Only one should be used if in the range of another one. That could cause commands ping ponging between them.
When I looked. The JV Digital Engineering XTBR plug in signal repeater was still available.