1939 M20. There is a lug cast onto the front fork on the right side just ahead of the tensioning knob. 1/2" ID with threads. What's this for? I would attach a picture but I can't seem to figure out how to do it.
Early in the production history of the M20, the front brake cable connected to a steel rod that actuated the front wheel brake. The cable and rod arrangement joined at this fitting on the front fork via an adjuster mechanism that mounted through the threaded lug.
After the rod- actuated brake was discontinued, the lug continued to be installed on the fork leg, but only served as an eye for the brake cable routing.