1FT - Word Manufacturer Identifier Code - in this instance Ford Motor Company USA, completed vehicle
E - the vehicle gross weight rating (and E isn't in my code sheet)
X15 - model code in this case F150 Super Cab
N - Engine code
6 - is a check digit
S - manufacturing plant - Allen Park, Michigan
K - ??
B12417 - Serial number
There is also a secondary label or plate that lists the option. paint and trim codes.
Hope this helps.