Hood numbers

[M2nut]

Hello everyone.   I'm wondering if anyone can tell me the logic behind the hood numbers on m2 cletracs.   I'm puzzled why the data plate ordnance numbers are 3 to 4 digits  and the hood numbers are 6 digits ?  There are  a few period photos that some early cletracs that have 5 digits painted on the seat rail side .  Almost all period photos have the first digit being a 9, which probably was the serial number range assigned to the type?  Any ideas?