These numbers supposedly started with 1001 and progressed from there, starting with 2001 at the beginning of 1964.
Often these specimens produced between October 1962 and December 1964 had an offset chassis.
If you have absolutely no idea whether your Marshall is old or new, a good place to start is the model name or series.
This will give you an approximate idea of the era during which your amp was manufactured.
Note: "A" Date Code: The "A" Date Code ran for 18 months (July 1969 - December 1970) so the "B" Code was not used.