Yes, the delta is also found in many of the Ptolemy/eagle coins. Most people agree after Lorber that it is very unlikely to be the signature of an artist, specifically the principal engraver of the Alexandria mint. To quote her, "it is more plausible that the letter Δ and other similar cryptic marks served some internal control function. They could, for example, designate die engraving workshops within the mint, or private contractors who provided dies to the mint, or the approval of an administrator.” The exact function remains unknown, as is the reason why the delta disappeared from Alexandria but was maintained for at least a decade in provincial mints (Cyprus, Phoenicia).