The Upper moon were numbered 1 through 17 with cancelation devices 17 through 20 held in reserve.
A Roman numeral I device was to be used for air mail letters
For ordinary mail, postmarks have C in the lower moon.
During Christmas time and Easter time "big ring" devices with AC, GS, SC, P, and M in the lower moon would be used in both even number and odd numbered years. Cancels Cancelations were for air mail letters.