80MC2M, 80MC2F. (microphone connector)

The 80MC2 connector is a balanced threaded connector, M or F indicate sex. I'm not sure of the history, but it would make sense to me that this was a precursor to the XLR, which proved a more popular format. I've seen this connector filling in as microphone connector on old Heathkit equipment, used as a filament voltage connector and as an output for electrodes.

Benefits:
Simple.

Drawbacks:
Uncommon.
Expensive.
No native jack switching to allow normalization.

The mounting hole for these is smaller than XLR and larger than 1/4". I had to remove material to fit the XLR, making it a non reversible modification.