1) Combining the AB165 and AA864 The schematics for this amp is explained below.
Few other models experienced so many changes in terms of looks and tone.
The bassman was a combo amp in the tweed era with the 1×15″ and 4×10″ speaker configurations.
From the blonde era Fender made the Bassman as a amp head and 2×12″ closed cabinet. The video below compares a Bassman AB165 to a Princeton Reverb and a Deluxe Reverb.
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 NBF theory is that you take the signal from the speaker output, let it go through a resistor and mix it in at the entry point of the phase inverter.