Glad to see another M3 up & running. They were a popular ENG/EFP model next to the JVC KY-2000 and Ikegami ITC-730 in a smaller package, the JVC used a dichroic mirror layout and magnetic deflection while the Sony used electrostatic deflection and a prism for color splitting giving it a big advantage in light sensitivity, registration stability and real detail. My M3a recently went to another member here and I still have my KY2700 with its mushy tubes. I did a write-up of the M3 in the Videokarma television Broadcast Gear section
https://videokarma.org/showthread.php?t=275482 and covered several of the issues with these.
The 4-pin DIN is the same as what is used with the VO-4800 portable UMatic, Sony wanted to standardize this until they got with the 4-pin XLR program. The DIN connector is a pain and I'd recommend changing over to the XLR staying with the standard of Pin-1 being ground and Pin-4 the hot B+.
Anyway congrats on the M3, I have over 30 years of experience with this one, if I get really bored I may post a few more Tricaster caps of some tapes shot with the M3 for your entertainment.