Could be a drawknob from an organ, but I am not familiar with what markings to expect on them. Most I have seen online aren't simply letters, but that does not mean anything.
organ "knobs" attached to shafts that are pulled out for changing tune and pitch on organs are called "stops" -- my wife is an organist. at church --also music teacher by profession. "F" is a musicial note / tone. -- yes its a rather old organ stop / "knob" -- organs were very costly items thus the nice mother of pearl touch on the knob .