The letter B was skipped because the letter A was used for 18 months and the letters I, O and Q were also skipped to avoid any confusion being mistaken for numbers.
Before starting, I must warn you that acquiring vintage gear can be really risky.
You can end up buying something that is not what you expected or even something that is damaged or modded beyond repair.
I won’t write the rest because it may confuse you even more. It’s important to note that, for example, a 1959 amp has nothing to do with the The JTM-45 JTM-45 with “block” logo The first Marshall ever made. The front panel has “Presence”, “Bass”, “Middle” and “Treble” controls, as well as 2 volumes and 4 inputs.
It was basically a copy of the 1959 Fender Bassman. In 1965, the plexiglass faceplates were introduced.
The table below outlines the meaning of these letter codes.