MSD with OEM Electronic Distributor

That's what it was! Doesn't the MSD give you their plug though so you can do that without having to go buy anything else?
IIRC at one time they did supply an extra connector which could be splice in either as a junction or replacement. That's what I was using for a long time. I just don't recall if it came in the kit. I guess I could pull the instructions out of the file...