I will take a look. If one cannot be found, a possible fix is to use a plastic repair compound to redo the back of the knob.. Antique Electronic Supply (
www.tubesandmore.com) carries this. I believe that theirs is called "Plastipare". I have used it to succesfully replace chips in painted bakelite radio cases.
Another note is that these usually have spring metal inserts. If this is missing, the knob will be loose and fall off easily. These can be taken from spare knobs, etc.