During the different eras the Bassman amp came in many different shapes and configurations.
At the Fender factory they used old tube charts when new models came, probably because the tube layout was the same.
Determining production date by serial number and transformer codes is better, and you should also inspect the circuit to be completly sure.
The picture below shows a 1969 transition model amp.
circuitry went virtually unchanged throughout its 19 year history.
So in theory amps made in different years will still sound pretty much the same. In its quest to produce cleaner-sounding amps, Fender in the 1970’s upped the internal voltage of many of its amps, the being one of these.
The silverface amps in general have a slightly different tone than their blackface counterparts.Silverface amps are cleaner and don’t distort as much.For the most part the differences between blackface and silverface amps are subtle.The age, amount of use, and type of speaker (if replaced), as well as capacitor drift, and type and age of the tubes all have an impact on the tone.Production years 1964 -1967 “blackface” circuits AA864, AA165, AB165 1967 -1977 “silverface” circuits AA165, AB165, AA270, AA371, AA568, AC568, AA864 Tube layout AA864 Tube layout (Seen from behind, V1 is to the right side): V1 12ax7 = Preamp bass channel V2 12ax7 = 2’nd gain stage bass channel (and normal channel for AB165) V3 12ax7 = Preamp normal channel V4 12at7 = Phase inverter V5 6L6 = Power tube #1 V6 6L6 = Power tube #2 Summary The Fender Bassman is a legendary guitar amp known to both guitar and bass players.It was introduced in 1951, primarily targeted for bass guitar players and promoted as a bass amp for the Fender Precision Bass guitar, the first mass-produced electric bass guitar ever.