Kenwood has the pin assignments available on their web site. (www.kenwood.net
.) They give the pinouts for mics, and many of their accessory cables/plugs.
The "Standard" Kenwood 8-Pin assignment:
1 Mic audio
2 PTT hot line
3 DOWN button (not applicable in your case)
4 UP Button -- ditto --
5 8 Volts, limited to 8 mA max draw
6 No connection; sometimes connected to receiver audio out in some radios. (Easy for Packet TNC connections!)
7 Mic (audio) shield/ground
8 PTT ground.
So you would use Pins 1,and 7 for mic audio, and 2 and 8 for the PTT switch on the MC-50.
Not sure how the original MC-50 4 pin was wired, but that shouldn't be too difficult to figure out, or get from the Kenwood site.
P.S. If sent by e-mail, the info might help you. Posted here, with the question, it may help others also.