This article outlines how to establish the date of manufacture for a Fender Twin Reverb amplifier.
Check the faceplate. Of the amplifiers labeled "Twin Reverb" (opposed to Super Twin Reverb or Twin Reverb II), there are two eras. Amps from the blackface era, which have a black faceplate, were manufactured from 1963 to 1967. Amps from the silverface era, which have a silver faceplate, were manufactured from 1968 to 1982. In 1992, Fender began producing the '65 Reissue Twin Reverb.
Find the tube location. According to Fender, amplifiers produced through 1969 have a sticker near the tubes. This is commonly stamped with a two-letter date code.
The first letter indicates the year: M 1963 N 1964 O 1965 P 1966 Q 1967 R 1968 S 1989
The Second Letter indicates the month: A January B February C March D April E May F June G July H August I October J September K November L December
Check the EIA number. Fender recommends dating unstamped amps by the EIA number on the transformers. The format is always EIA 606-Y-XX or 606-YY-XX where Y indicates the year (196Y/197Y or 19YY respectively) and XX the week.
Try the serial number. Dating by serial number, while less accurate, can be of some use. Stamped on the metal backing of most amps is the serial and production number. It is typically in the A00000 format.
The following list, compiled by Greg Gagliano, establishes dates through 1976: A00100 to A01200 - 1964 A01200 to A04300 - 1965 A04300 to A07000 - 1966 A07000 to A10400 - 1967 A10500 to A11300 - 1967 A10500 to A16500 - 1968 A16500 to A21400 - 1969 A21400 to A25600 - 1970 A25600 to A37000 - 1971 A37000 to A50500 - 1972 A50500 to A68000 - 1973 A68000 to A99999 - 1974 A81000 to A99999 - 1975 B01000 to B15000 - 1975 B15000 to B68000 - 1976
If these steps are unsuccessful, a number of books have been published that help to date amps based on part codes. Some are listed on Fender's website.