Pressing on...
I've now tested this large 5"x7" meter. It is indeed 50uA FSD, with a resistance of 3.9k.
Thinking about it some more, that large scale would probably be under utilized just for use in an ESR meter, but then it occurred to me that since a scale has to be created in any case, one could make better use of its large size with multiple scales. It could be part of a more comprehensive capacitance tester, measuring not only ESR, but also leakage and capacitance, with scales for each, and possibly also show leakage test voltage.
Just thinking aloud about how that might work:-
A rotary switch to select the required test function, and a "test" button to perform it. When not pressed, a relay would connect the DUT to a discharge circuit to ensure it's safe to test, and when pressed, switch over to the testing function. For example, in leakage test mode, while test button not pressed, the meter would show the test voltage (and able to be adjusted by a pot), and then show the leakage current while the test button was active.
Any thoughts on that idea?
Also, I'm still interested in hearing of any other possible uses for such a large analog microammeter...