How do you read the wiring diagrams for the 73 Standard Cluster?

[WH23G3G]

I got my dash frame and cap out in about 30 minutes, everything was already off of it. There was one screw on the driver's side bottom corner that had 3 ground wires secured with a sheet metal screw, that I tried for over an hour to unscrew and could not get it to break loose. So I had to cut the wires. Other than being a little dusty in the fuse block, the dash harness looks great. What I'm wanting to do while the dash is out being repainted is to go through all the plugs and label them as to what they plug into. Layed out on my floor it doesn't look like all that much. Some I know what go to, but a lot of the stuff wasn't even hooked up when I got the car. I'm referring to the 73 factory chassis wiring diagrams, but I don't know how to read them. I go to a plug on the harness look at how many wires it has going into it and the colors and where it is in relation to dash and try to figure what it could be by the wiring diagram. But some of the symbols aren't explained in the 73 electrical section or I missed them. If I knew how to read the letters then I could probably figure out all the plugs. For example right now I'm looking at the furthest wire on the passenger side which uses a single wire male bullet connector, which I'm assuming is for the door jamb switch on the passenger door. It has a yellow wire going to the male bullet connector. On the factory wiring diagram it shows The right front door automatic switch and even shows the bullet connector and it labels as M2C20Y. Now obviously the Y is for yellow, what is the M2C20 for? Is that just to trace the wire on the diagram so you can find out where it begins?

[Plumcrazy]

The "M2C" is the circuit identification number and the "20" is the gauge of the wire used in that circuit.   

[70 Charger RT]

Interesting.  Learn something everyday.