You are correct - that is the casting number of the main case for a 1968-1970 Muncie transmission.
The trans serial number is actually the date code. It will also have the VIN of the car it was first installed into stamped on a machined pad at the front of the main case. I am not positive - but I am pretty sure - that none of the castings used in a Muncie trans have date codes. The trans will have a date code (serial number) based on when it was assembled. This is also stamped into the main case, on the opposite side of the case from the VIN. (? ... I wish I had mine to look at to be sure.)
Here's a cool site that should help you tell what you're looking for:
