3 KudoZ points awarded for this answer. Comment: "I ended up using "Acting Director". From another source I found out that the "ACC" stood for "accidental", and it was only then I could have real confidence in the meaning. I do medical translations, and although this term wasn't medical, I am used to having to completely understand the original; otherwise I have to tell my client, "It could be A, or B, or C, or something else," or even the dreaded,, "I don't know". Still, although I had started out suspecting the answer, your experience as a native speaker did help me keep looking in the right direction, so I definitely owe you many thanks."