Early Marshall through the JCM800 series July of 1969 Marshall introduced the date coding system.
Some of the older amps have an inspection sticker on the top of the chassis and they usually have the day, month and year the amp was made or inspected.
The switch from 6L6 to the KT66 valve gave Marshall amps a more aggressive tone.
Eventually the KT66 became too expensive causing the jump to EL34 and even more aggression. [Source: Wikipedia] Dating Marshall cabs and amps Info from Wayback Machine Serial number (no barcode) version: Marshall Serial Numbers and Chassis Codes Does it have a serial number stamped on the chassis?
1979-1981: Some Marshalls made from 1979 to 1981 have the serial 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.
The new system designated the year of manufacture in the first two digits of the number and the week of manufacture in the last two.
Marshall amps began featuring aluminum back panels in 1969.
Serial numbers began appearing on the back panels in July 1969.