Here is an explanation that comes in handy with our 68 pic examples John.
"The code consists of two circles, both 1/2 inch in diameter and divided in half. One circle is called the date marker, the other the status marker. The date marker has a month designator for when the part was cast or manufactured. This is represented by the numbers 1 through 12 (Jan-Dec) in the years 1963-1966. They were marked with letters as shown in the table below from then on. The opposite side of the date marker has between 1 and 5 dots representing the week of the month. The easiest way to discern the proper date mark circle is to look for the week designator spots. Always check to see that the date marked precedes the final assembly stamp code. The status marker has the latest blueprint change number on one side of the circle and a deviation notice letter on the other side if the part change letter is active." This information was only useful to the Muncie plant by allowing the tracking of problem parts. This circle is usually blank." Our 67 & 68 M21's still have #'s for the month......
Here is where the above info resides- http://members.tripod.com/camaro_obsession/muncie.htm
And a picture of our bell housing and our trans tail housing casting date.
403 cast dates a little funky but believe to be 3rd week of Oct. as it has 3 defined tits/bumps.
Tailhousing is 1st week in December. By the way, trans was built 12/29/67. Notice the other circle (status marker).
Excuse the grime as pics were taken for documenting and before cleaning/restoration.