A friend of mine showed up with this thing the other day. I traded him some stuff for it and started researching.
The serial number is 37636 and the attack pot dates to the first week of 1968. (1376801). And since they made about a gazillion of these things I'm guessing that pot didn't stay in the factory for very long, so I'm going to assume this left the Kalamazoo factory in the earlier part of 1968.
A bunch of things on this pedal have been changed/lost/serviced.
First and most importantly* the knobs.
(*since looks really is the only thing that really matters.)
Since these pedals used the same type of knobs as the gibson guitars of the era I'm sadly assuming a previous owner put the original knobs up on ebay claiming they came from a 1968 SG Custom or something ridiculous. The same way it seems to be hard to find a 50's or 60's fender or gibson lap steel with it's original knobs. Or sometimes even pickups.
I've been trying to figure out what knobs this one would've had. I couldn't find any specific information about it anywhere. But from my research I feel somewhat confident in saying that this one probably had the gold reflector knobs. As that is what I've seen on all the others with similar serial numbers.
So my guess is that Gibson changed from the black reflector knobs to the gold ones in 1968 or late 1967. Again, I'm really not sure and I have no facts to back this up. Just some observations.
(I've also seen a couple of FZ's with the "witch hat" knobs. But it's hard to tell if they would be "original". Since those knobs nowadays are really hard to come by it would be strange for someone to waste them on a pedal, but oh well. Who cares?
I wouldn't be surprised if they just grabbed whatever knobs they had at the factory if they ran out of the ones they were supposed to use.)
Here are some more pictures of famous people using the FZ1a's predecessor the FZ1.
Black knobs: 1965-67?
Gold knobs: 1968?
Anyway, I don't know what I'm talking about.