So, unfortunately, having a serial number is of no benefit in identifying the year of a particular amp.
How to read the serial number (1969-1983) Marshall used a coding system that provided (a) model, (b) serial number, and (c) manufacture date.
Other sources to date your amp may include serial numbers and overall look and visual components on the amp as well.
For example, the logo on the amp or materials used during a certain time period of production.
(c) Manufacture date - The last character determines the date of manufacture.
Since A was used in both 19, Marshall decided that 1971 would start with C.
Note that each different period is approximately during each different ownership. Pre 1953 - These amps can only be dated by the EIA (Electronics Industries Association) codes on the speakers and other parts. This was a six-digit number like this: YMMNNN, which was the last digit of the year, month, and number of production.