Thanks, guys.

I am going to guess that the "D" stands for the model year, i.e. 1968 in this case.

I have a 1965 383 and the top line reads A 383 so I am guessing again that Chrysler started using the alphabet to stamp their blocks in model year 1965.

I also have a 1974(?) 440 and the top line of the stamping reads "6T440E". WTH does that mean?