Only just spotted this thread, so I don't know whether people are still looking for info. I'm referring to my well-thumbed copy of Gruhn's Guide to Vintage Guitars and under "Fender Acoustic Flat Tops" (which I assume is what everybody above is talking about?) it says that the F-series was introduced by 1969 and discontinued by 1979. Here are the serial numbers / dates Gruhn gives for the F-series (note that there's some overlap between years):

100173-147397 : 1965
115151-215697 : 1966
180882-230599 : 1967
213820-262774 : 1968
224160-290835 : 1969
293780-304089 : 1970
295668-331031 : 1971
315451-375648 : 1972
362789-588161 : 1973
538585-615662 : 1974
608004-717257 : 1975

Hope that helps!

Sarah McQuaid

www.sarahmcquaid.com