Marshall used letters as a date code to represent year of production. The letter B was skipped, as were the letters I, O and Q since they might be confused for numbers.

What setting should my amp be on?

Clean Tones Then go for around 5 for your bass, and 6 for your mids and treble. To give your tone a bit more definition and presence, consider adding a reverb pedal if your amp doesnt have this setting. Reverb pedals give an echo effect which sounds great on lead guitars.