So I stumbled on a research paper by Okuda and Takedatsu  that uses mμg as a symbol for the mass unit.
I couldn't find anything about this measure unit anywhere except a recommendation not to use it. So how much is 1 mμg? I suppose it means milli-micro-grams, so it should be 1 μg/1000, but it's just a guess.
Okuda, K.; Takedatsu, H. Absorption of Vitamin B12 in a Rectal Suppository. Experimental Biology and Medicine 1966, 123 (2), 504–506. DOI: 10.3181/00379727-123-31527.