Mercury shutters were simple, and usually reliable and accurate. The rear pivot for the shutter is visible just above the film gate. The front pivot is less accessible. Perhaps cleaning these pivots will improve shutter function. The extinction meters relied on a photographic step wedge which has sometimes deteriorated. The OP's extinction meter appears to be the prewar version, not the somewhat similar postwar model. These 40,00 prewar cameras are less practical than the more plentiful postwar Mercury II because of the early proprietary film cassettes. They do retain their appearance better due to the aluminum body and leather trim. After the war Mercury changed to synthetic leather and a magnesium and aluminum alloy that tends to corrode. At least the OP has the often missing take-up spool. I bought a used Mercury II for $20 in 1950 on a $75/month Navy salary, lost it, and bought another for $20. The half-frame image was inconvenient, so a more conventional camera was the next upgrade.