AN-6-00 to FN-6-00 fronts

This series has AN-6-00 to FN-6-00 printed in the lower right hand corner, followed by either a blank,
or a series of letters always starting at "K" then descending in sequence to "A".
Once "A" is reached, a series of numbers always starting at 9 then descending in sequence to 1 follows.
Earliest known date: 30 MAR 01.