SM124 is 71Hz. There is no multi-sync system and it won't live much time at any other frequencies.
The B&W is taken out from only one pin of the plug. I doubt there is any use of the RGB pins while an SM124 is plugged. The stream is rather a square bit stream than an analog stream.
So I suppose you will have to force your PC output to work only at 71Hz and convert the RBG signal to square bit stream with a latch or a trigger.
