The newest models are incredibly easy to date (read on, if you aren't already aware of the current serial numbering system) but dating older amps (pre-1993) are slightly more involved. Here's the story so far…  

July 1969 - September 1992  

In July of '69, Marshall introduced their first date code system. To determine the year of manufacture, a letter of the alphabet was included in the serial number. Here's how it works:

A – 1969/1970       C – 1971               D – 1972               E – 1973

F – 1974               G – 1975               H – 1976               J – 1977

K – 1978               L – 1979               M – 1980               N – 1981

P – 1982               R – 1983               S – 1984               T – 1985

U – 1986               V – 1987               W – 1988              X – 1989

Y – 1990               Z – 1991/1992

“B” was skipped because “A” ran for 18 months and “I,” “O” and “Q” were also skipped because they looked too similar to numbers.

As a rule, the serial number is usually located on the back panel of the chassis but some amplifiers made between 1979 - 1980 had them on the front panel. Now, at this point you're probably wondering where exactly this letter can be found within the serial number. That's a very good question so here's the answer. From July 1969 to December 1983, the letter was put after the serial number. For example, let's say you found this stamped on the back panel:

SL/A  14532  D

What does it mean?  

SL/A = model number

12532 = serial number

D = Date code (1972)