M 136394 series

This series has M 136394 printed in the lower right hand corner, a space, then the letters "A", "B" or "C",
a space, then a series of numbers (commencing from the right and working inwardly towards the "A", "B" or "C")
always starting at 5 then descending in sequence to 0 (zero).
Earliest known date: 15 FEB 97.