school me on dist

I was able to tear into some dist today.

From left to right are

New/re-manufactured 78 Truck with a R 13 Governor
Dist I removed from a 74 Dart with a 9R Governor
New/re-manufactured 78 Truck with a 9R Governor
Random Points dist that I picked up with an L13 Governor

Can someone explain to me how these numbers on the Governors are read, I know its times 2 to get a total reading cause cam turns at 1/2 crankshaft speed but how did they come to this number.

From what I have read it has to do with the length of the slots on the governor....OK I get that, I see that but for instance evidently from what I have read a long slot is a **** Governor cause it allows to much advance which is something to do with the emissions deal.

Id like to understand all of this better.

Id like not only advice on what might be the best Governor I should be looking for that works best on my own heavy 4 wheel drive truck but why I need this Governor.

I am not looking for someone to tell me what I need only but help me to understand why I need it.

Id like to get into these slants and I dont want to have to come running on-line every time I have to make a move with my ignition cause I was too lazy to ask the right questions.