It takes longer to tell about how to use this than to actually perform measurements. Once you have been through the process a couple of times, it only takes a couple of minutes to fully investigate a meters characteristics.
Place the unknown meter between C and D, The orientation needs to be so that positive voltage applied makes the meter move, most normal meters that means the positive lead connects to C. I have a couple of centre zero meters as well so the connection could be either way. With the battery connected select full scale on the switch position.
Rotate R1 for maximum resistance. Many pots have maximum resistance (which will be minimum current) when the control is fully CCW, but the direction of travel is arbitrary and unimportant. The important thing is to be sure it is set for minimum current to start the test.
If you have it set your DVM to read 200 mA, or more, then insert it into the circuit with positive lead to B and negative lead to A. Always start with a high current setting on your DVM and work down.
You may see absolutely nothing on the meter, adjust R1 (reducing it) to obtain a full scale reading on the unknown meter by reducing the resistance. If a full-scale reading cannot be obtained with R1 set for minimum resistance, the unknown meter is beyond the current supply capabilities of the batteries. In such an event, disconnect the DVM quickly to prevent draining your batteries. This condition is probably due to an internal shunt in the unknown meter. This can be overcome as will be explained, later.
As I was wanting to use mine as an audio level S meter for the Norcal-20 O built (maybe a good idea?) I wanted a meter with FSD of between 100uA and 500uA. The first meter I tested has a full deflection current of 200 uA.
Now re-adjust R1 for a full scale reading.
Change the S1 to Half Scale and adjust R2 for a half scale reading.
YOU NOW KNOW THE INTERNAL RESISTANCE OF YOUR (formerly) UNKNOWN METER.
Given that you are salvaging a meter, the next stage should not be a problem, surgery is required. That is, you must open the meter and remove the built-in shunt, then proceed as you would with a "normal" meter. The internal components are mechanically very delicate, similar to the insides of a mechanical watch. So, if you have the patience and dedication required to disassemble the meter, remove the shunt, re-assemble the meter and perform the Meter Master investigation as described above, you can salvage most any meter. Some are very easy to get into some seem to be sealed within plastic, your milage may vary and may not always be successful, but it will be a satisfying end if you are so inclined. If not, its best to simply toss the meter into the nearest trash can and forget about it.