4C/M9 is the Burman casting number for the kickstart case cover. All of these cases are number 4 the 'C' denotes C type gearbox that we usually refer to as 'CP', which actually means 'C' type gearbox with a pivot mounting arrangement, 'M' is short for 'Mark' as in version and the 9 is well the 9th version. The W/NG parts book for 1941 gives the Ariel number as 3023-41
I have never seen the other number before, I suspect it is related to a repair or replacement with the date of same.
Worldwide Membership Secretary
Semi-retired partner in Draganfly Motor Cycles