The differences between pre and post CBS did not occur overnight. They didn’t immediately end manufacturing and switch sources or make dramatic circuit changes for no reason. Most late blackface used the same components as silver face and silver face used mostly the same blackface parts. Circuit changes were mostly done to accommodate newer part specs or to save money in production. Having been a truck for 50 years it’s typically the engineer at the time initiating improvements based on amp failures seen coming in for repairs. Eventually cheaper parts and lower cost employees building gear invites mistakes and damages sales due to gear performance or failure. Fenders are simple and cheap to upgrade so any that have the basics like pots and transformers in good shape can be refurbished to run like new is you lay the hocus pocus to rest and focus on the basics.

The guy with the “1966” Bassman with the AA864 probably has a 1964 and does not know it. He thinks its a 66 due to the “Patented 66” on the front panel. Its a common mistake. He should verify the chassis serial number, and internal stamp along with the pots and serial numbers on the transformers.

Ah, the fuzzy gray years between sale and handover. Or if your really that keen you need to date your amp to DD/MM/YYYY, the ‘What’s on the shelf’ period. Leo, bless him, was a practical man. He designed the best amps in the world for as cheap as he could. There has been lots written about what was going on at this stage at Fender, but basically be happy you have a Blackface. Trying to date them is a joke. And the secret is not in the board, it’s in the circuit.
Happy owner of a nominally ’66 Bassman.
Cheers, John.
PS My sticker says AB 165. It’s not, it’s a AA 165. Just using up those AB stickers before the sale instead of ordering new ones for AA 165. Either that or they ran out of AB 165 boards and had plenty of AA’s left.