Serial numbers have been used in various locations on Fender instruments through the years.
They have been placed at the top of the neck plate, on the front of the headstock, on the back of the headstock, and on the back of the neck near where the neck bolts onto the body.
They were stamped on the back vibrato cover plate (early '50s Strats), and on the bridge plate between the pickup and the saddles on some Telecasters.
But once again, due to the modular nature of Fender's production methods, and the fact that most serial numbers schemes are not sequential and usually overlap from between 2 to 4 years, (from the early days of Fender, through to the mid 1980s), dating by the serial number is not an exact science.
Given the modular nature of Fender's production techniques, an individual neck may have been produced in a given year, placed in the manufacturing warehouse and remained in stock for a period of time, and then subsequently paired with a body to create a complete guitar in the following year.
So, obviously a neck date, while being helpful in providing a date range of production, it cannot be a definitive reference.
You will notice that there is quite a bit of overlap of both numbers and years.
The following charts detail the most common Fender serial number schemes used from 1976 to the present.
The letter B was skipped because the letter A was used for 18 months and the letters I, O and Q were also skipped to avoid any confusion being mistaken for numbers.
The serial number on a Marshall amp is usually located on the back panel of the chassis but some amplifiers made between 19 had the number on the front panel.
For example: S/L C 12357 S/L would indicate the model Super Lead 100 watt C indicates the year 1971 12357 is the serial number. Starting in 1993 until 1997 Marshall switched over to a new serial number system.