Well, let's compile the data. I dug out my tank and added it to the mix.
12D 68 20
02C 68 27
07D 68 52
01B 21 69
01C 22 69
05A 37 69
06A 24 69
I think the code is a running week date, starting at 01 about August and incrementing throughout the model year.
More data would help. Anyone with an early 68 or early 69 car (Aug-Dec) that can add their tank date?