First question - What was the gear ratio? - when was it assembled (from the tag)? - Open or closed?
Never tried to figure out what everything "means" To hard and too likely IMHO a failing effort since we have so few clear examples and no eye witness data to start with. Kind of like reconstructing a whole dinosaur from a single toe bone. We've seen how well that has worked in that community over the years
1- Believe the "Y1" would be "41" and assemblers id or inspectors - that high of a number I'm beating its an assembler. Your "9D" might be "90" Not easy to write with a bush given certain angles, limited space, rough surface and the push to get it done quickly
2- Looks like you have a fill indicator near/at the filler plug.
3- Typical coding on the bottom of the third member for an early 69 - moved to the rearend housing at items and later so likely that it was added after the whole assembly was together.
4- Looks to be some of the typical assemblers id numbers at the top of the case (above the 1st rib)
5- Would guess that the "extra" red oxide was applied to erase what ever was written below it - not a typical thing IMHO found on 3rd members
Hope this helps a little - will look at it again once we get the additional information
