I am working with a 60 year old schematic and test procedure. The latter refers to adjusting rheostats to "300 DCMI" or "between 100 and 130 DCMI". There are voltage and ammeters attached to different terminals. Typical usage in the document would be:
Adjust the rheostat RA for 50 DCMI indication on meter A1.
(Meter A1 in the drawing is in series with the signal, so it is almost certainly a current meter, but this is not explicitly called out.)
The procedure is rather vague, but I'm almost certain that it's referring dc mA. That said, the procedure was probably written by a British firm and I'm wondering if DCMI means something else entirely or if there are subtleties I'm not aware of.