AT means remanufactored parts. In german: "Austauch Teil" or something like that. Normally it was controllers that were repaired and sold a bit cheaper than a new part. Today, I think, it is mostly used to be sure the old part is returned and thereby taken out of market. Earlier there was a big market for older controllers/cores. Some repairing companies bought in all they could get, and sold them afterwards.
Regards, Max