You'll need to clear the fault first. Whatever is shorting your mic with 150 Ohms is pulling down the current in the 21A mic, but allowing sufficient current to flow thrugh the carbon mic. When we were kids we would wire a couple of handsets in series mic-rec -some wire-battery-some more wire-rec-mic and we'd have an intercom. Wrong thing to do (because of the need to keep d.c. out of the receiver), but it works.
Right... I've just had a look in the few telephones I have with Handsets 184. The metal plate behind the centre pintle does indeed say 'M/R2'.
But this only applies on a handset 164 and is most probably down to economies of manufacture and interchangeability (telephone manufacturers are very good at that).
However... Rest assured that, on your handset, the two receiver terminals are the ones marked R1 and R2 in the bakelite. The two microphone terminals are the centre pintle and plate, and M1 in the bakelite. The two mic terminals marked M/R2 and M1 are not in any way connected inside the handset to the two receiver terminals marked R1 and R2.
As I said earlier, it looks like the common connection to mic and rec has been made from the green wire on yours.Attached, for your confusion (
) is how a 164 handset connection looks, where the common connector
is the centre pintle.
I wouldn't worry about a spare 0.1uF capacitor. Your telephone will work fine without one.