Does DCMI mean dc mA?
In this context, I believe DCMI stands for "direct current moving iron", which is how the meter measures and displays the voltage or current measurement value. This book provides some more detail about these types of meters: 


  The current to be measured, in general, is passed through a coil of
  wire in the moving-iron instruments. In case of a voltage measurement,
  the current which is proportional to the voltage is measured. The
  number of turns of the coil depends upon a current to be passed
  through it. For operation of the instrument, a certain number of
  ampere turns is required. These ampere turns can be produced by the
  product of a few turns and large current or reverse.


And this article provides some additional diagrams and information that may be useful.

Without seeing the schematic or procedure, it will be difficult to confirm whether it means 300 mA DC (as opposed to 300 uA DC or 300 mv DC).