We don't track the preamps like we do the instruments, afterall, they aren't usually very custom. But, you should be able to find a handwritten 4-digit number inside on either one of the pots or on one of the boards. This will reveal the year and week the preamp was built.
The city name on the back and the case description can get you a rough estimate of the age. From your description, Bob, your F-1X is probably from around 1990-1992. Your red capped knobs are the same as the current knobs, only with red caps.
For Edwin's, I wouldn't assume it was a prototype - even those get serial numbers in our way of doing things.
BTW, these red capped knobs are not the red capped knobs on the oldest F-2B preamps.