I also remember seeing the small F variety on a few of the prefixes on the Birds $5s. Prefixes I definitely recall seeing were FNA in the 7 million range, FNE in the 9 million range, and FOS in the 9 million range. I may have seen one FP* banknote with the small F (FPU?), but I don't remember offhand.
The small F variety would not become standard until somewhere in FHG or FHH on the $50s in 1995 - after the $5s entered printer letter G (which always was the standard small G), though some higher FNX replacement notes did have the small F (7 million to 8 million range). Just before the Journey series launched in 2001, the $50 was the only banknote using printer letter F at the time.
The big F - used in Canada from 1988 through 1994/95 - looks a lot like the F used in the serial numbers on U.S. currency, and same for the big B on Canadian $1 and $2 bills in the late 1980s.
I wouldn't be surprised if FDR also got released only in limited quantities during the final week or two of the multicoloured series, as I don't remember seeing much of that prefix in ranges outside 1.3M-1.4M (I recall seeing one in the 8M range, but nothing else otherwise). Makes me wonder if FDR was much scarcer than FDS?
