These are still found on stickers with a bar code on the back of the amp.
It's important to note that, prior to 1984, this letter came last.
The letter B was skipped, as were the letters I, O and Q since they might be confused for numbers.
They were initially broken down into three parts: the model code, the serial production number and the date code.
Marshall used letters as a date code to represent year of production.
Starting in August 1997, Marshall moved to their current serial number scheme, which is easier to understand and gives more information than previous schemes.