Now, we obviously haven't looked at every amp made in January 1966 so this isn't set in stone.
If anyone has an amp with a "PA" stamp, please let us know.
Greg is a long time Fender collector who is very knowledgeable not only in the details, but in the circuitry as well.
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.
In the fall of 1965, Fender switched from stamping these numbers in black ink, to dark green ink.
These 1965 codes begin with the letter "O." The code for the year 1966 is "P." However, someone in the factory apparently forgot to switch the stamper from "O" to "P" in January 1966.
After a quick websearch of "Fender Twin Reverb Date Code", I found a few interesting breakdowns of date and production codes, but couldn't decipher the number you have.
Do you have a picture of the label with this number? I ask because there are more tube reference numbers than production date codes. Lutherluke hiwhile dating is not supposed to be as accurate with serial number as with date stamp on the label or EOI number on the transformer- the serial number you give is i think 1973see bottom of this page help?