I added it, but you’ll now note there are two F1 entries and also two DN entries.
But it gets messier as of just now: whilst reading an old story on the Intel Developer Transition Kit, the serial shown is also prefixed as F1.

Though knowing Apple’s past history of using single-glyph location/factory identifiers in the very old days (including “C” for Cork, Ireland, well into the 2000s), I have a hunch the DTK — effectively prototypes — were assembled at the old Fremont factory (“F”) going back to the 1970s and 1980s.
“F1”, seen here, could be the way the third glyph position later on denoted which factory or assembly line within a location assembled the item. Without knowing for sure, it’s conjecture, but the following three digits — “536” — or, September 2005, sounds right about when one would expect a DTK to be readied.