120 is the length of the film 120 cm. 220 is 220 cm long. 135 (35mm) 36 exp is 135cm long
Nice theory, but...
They're just names. If Kodak had used letters, everyone would have wanted to know what the acronym was for (and made it up if they didn't get an answer). So they used numbers instead. Then everyone can wonder what the numbers mean, and make it up when they don't get an answer