Manfrotto has discontinued the part R190,311. This was a common center column locking knob for many Manfrotto and Bogen older style 190 tripods. You can use the following:
R190,41 is a similar replacement, it has a spring and the diameter and thread size is the same. Click here for more info.
R004,80 is the correct diameter and size, but there is no spring. Click here for more info.