I am just finishing up with detailing my C-4 for concours.
Lots of research has been done and reported. I even had input from people who worked in the aluminum case foundry.
Here's the skinny from what I have been told on the 3 case parts.
Each of the three were cast on different lines in the plant. When they were removed from the mold and still hot, the worker wrote his shift # about 2 inches high. A "1" = 11pm to 7am..."2" = 7am to 4pm, etc.
The middle case got deflashed and steel shot blasted in a Wheelabrator room or similar. There is no evidence that the tail cone or the bellhousing ever got shot blasted.
So the middle case was a shinier appearance than the other 2 parts.
Any or all of the 3 may have got selected to go to QC for extensive measurement and QC checking which most likely consisted of at least concentricity/dimensional checks, thickness measurements, and porosity checks. The worker said when the ones that went to QC came back, they had writing all over them.
The top of the mid case always got an "OK" stamped on it. All C-4's got a shot of orange spray paint on the center case and the tail cone. Some pictures from Charles Turner show a 4 or 5-digit number stamped on the center case using machinists RED layout dye.
Stacy [ivygreen65 on this forum] has some finished pictures of his. I will post some when I'm done.
YES...THEY ARE ALL NATURAL ALUMINUM COLOR. NO PAINT. TAKE A LOOK AT THIS THREAD C-4 Markings/Daubs