screw in bottom of Master Cylinder Reservoir?

In process of replacing my master cylinder and noticed a screw at the bottom of the reservoir for the rear brakes. Wasn't expecting it. Trying to figure out its purpose. New replacement MC doesn't have same...

Hole is threaded and screw has a short pin at the bottom after the threads.

Car is a 1972 Duster w/ manual disc in front, drums in rear.

Pic attached...

Anybody ever see this before?

Thanks,

-R