[Mr70]

Different service/replacement parts plants started/ended at different sequence numbers.
For these transmissions,I've only seen Muncie assembled cases within those ranges.
Here's some others:
Cleveland plant transmission assemblies were #00001-14999.
Toledo plant transmission assemblies were #15000-24999.
Saginaw plant transmission assemblies were #25000-34999.
Muncie plant transmission assemblies were #35000-44999.

At some plants,once they used up all the given numbers in a series,they went back to the beginning sequence number,but this time they placed the letter A in front of it,but understand not every plant did this.

I'm pretty certain the letter C designated CHEVROLET,and not Counter.The letter after it stood for the component,be it T for transmission,H for Hyrdra-matic or E for engine.The fact that these were also available over the counter was just a coincedence that it fit..

They even designated the Company they were destined to.
C was Chevrolet.
L was Oldsmobile.
K was Cadillac.
B was Buick.
P was Pontiac.
This applied to other service parts like partial engines,fitted cylinder cases,just cylinder cases,transmission assemblies & just their cases.

In Toms example,..CC9 A40010B...I've not seen a 1969 service case like that,so I wanted to be sure the 9 wasn't actually the number 0,like in Pats example.
Here's what I believe Toms to decode as:
C-Chevrolet
C-Case..they just swapped in his old guts,and did not spring for an entire new transmission.This was common as they did the least amount they had to under warranty.
9=last digit of year case was completed in.
A=reset back to the beginning of sequence.
40010=sequence #.
B=M-21 Transmission I believe...this is what's unusual to me as I thought this was only notated in 1970.

Pat,yes each calendar year,so yours is indicating it was assembled sometime between Jan.-Dec.1970.