I would say it was a 1975 or 1976. The first of the blue ones were stereo, had the big power switch and were labeled San Francisco, and those lovely red capped knobs.
I am not sure how the numbering on the F2B's started. I would love to know. I have one that is stamped 102 and was confirmed by Ron to be one of the first 10 made.
Supposedly identical to all the rest except that there is only 1 filter capacitor on the power supply. And yes, I can hear the 60hz hum. The other differences are that there is no LED on the front of the unit. Instead there is a Fenderesque lamp. It is strictly stereo with no mono output. It also does not have rack ears. The unit is grey with a cream color face. The knobs are the witch hat knobs from the era.
I also know that the schematic for the F2B changed after serial number 144. That is when they went from the Fender lamp to the front LED.
Check out this link:
http://www.furmanhistory.com/ Janet Furman talks about the F2B design and the original F2A and it's circuit problems. Maybe that is what I have with the pitiful filter in the power supply.
Michael