The 4x10 Bassman used the prefix "BM" while the piggyback Bassman used the prefix "BP." The reverb units have the prefix "R." In addition, the tweed Super, Pro and Bandmaster sometimes have the prefix "S" in the serial number.

We combined all of our information into a computerized database for this project and for the past 18 months have been slowly (sadly, very slowly) gathering information that we collect ourselves as well as from other people. What we need is the following: 1) Model name 2) Model number on the tube chart 3) Date code letters on the tube chart 4) Speaker codes (if speaker is original) 5) Transformer codes (if the amp doesn't have date codes on the tube chart) 6) Cosmetic features (flat/raised logo, tweed/tolex, blackface/silverface, rough/ smooth blond tolex, white/skirted knobs, TV-front/wide-panel, etc.) One very interesting and very important factoid has surfaced regarding the date code letters on the tube chart.

In the fall of 1965, Fender switched from stamping these numbers in black ink, to dark green ink.

The biggest tip off would be the control panels which brings us to interesting factoid #2.

After CBS bought Fender in January 1965, there were still plenty of control panels for various models that were in stock.

These say "Fender Electric Instruments." Depending on the model the use of these pre-CBS panels have been observed on amps as late as August 1965, except for Champs and Vibro Champs which had foil stickers on the back the chassis.

Fender must have had a million of these labels printed up since they appear on Champs and Vibro Champs well into 1966.This makes things a little more interesting and difficult in playing the "dating game." And yet there is another variable to contend with as it appears that models that share chassis may also share the same serialization scheme.For instance, the Bandmaster Reverb and Super Reverb share the same chassis, and the Twin Reverb, Quad Reverb, Dual Showman Reverb, Vibrosonic Reverb, and Super Six all share the same chassis.Likewise, the brown/blonde Tremolux, Concert, Vibrasonic, Twin, Pro, Super, Vibrolux, Showman, Dual Showman, and Bandmaster used a sequential numbering independent of model, but as with Fender guitars, these were not used consecutively. The 1956 to 1963 Champ, Harvard, Princeton, Deluxe; the 1956 to 1960 Vibrolux, the 1956 to 1964 Bassman, and all the tube reverb units have their own serialization scheme.The Champ has a "C" prefix, the Harvard an "H" prefix, the Princeton a "P" prefix, the Vibrolux a "F" prefix, and the Deluxe a "D" prefix.Jeff is an amp tech who specializes in Fender amps, i.e.