4 SPD trans (MUNCIE)

[Dicky]

Hi, and Happy New Year, I'm puzzled by the nomenclature of the transmissions used in vehicles. Example; there is a common 3 digit number that some how designates GM's Muncies in the 60's & 70's like 451, 452, 453. Where did/does
this come from? In my search I find that it includes a variety of trans, see this link:https://www.shifterdoc.com/4_Speed_T...n_ID_Chart.PDF Can't figure it, Thanks DH