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It is the case marking for a specific trans. Pick up a Ford Parts & Accessories book or CD and it's useful to help identifiy which trans you have should the tag on the trans go missing. (Our helpful in identifying any part for that matter.)
EG the pic below is a New Process 5spd o/d from a 1962 F-750 w/332 Y-block. The N5C code (faint in photo), shows up on the Ford Trans ID chart page in one of the columns.
I do not have a book or CD for your year span or I'd look it up for you. Pick up a book, they can be invaluable in helping decipher your truck and looking for parts.
Sorry, take that back, I had '66 on the brain. It looks like 4TE? If so, 4 spd, Shift type "C" (center / floor), 6.398:1 ratio, Trans case painted marking "4TE", Mfg No ASI-T98, hand brake drum, Warner Gear, part number B3C 7001-C. I printed and scanned a copy of the page and attached below.