Compact Disc Codes
 

Many of my discs early on had letter designators, and I don’t see that any more.

Any reason?

Any preference?

AAD - analog recording, analog mixing, digital transfer;
ADD - analog recording, digital mixing, digital transfer, and;
DDD - digital recording, digital mixing, digital transfer.

At some point in the nineties, they were all DDD, so there was not much to add by
having the markings, but that may not be true anymore. There are well known
Tacet recordings that are analog (even all tubes) and mastered to analog tapes.
The only digital aspect happens when porting to CD or SACDs.