AN-6-99 to FN-6-99 series

This series has AN-6-99 to FN-6-99 printed in the lower right hand corner followed by 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 0 (zero) then descending in sequence to 1 follows.
Earliest known date: 01 JAN 00.