I have a CTC-11 too!
The channel selector knob on my set was missing the center piece as well... Must be a pretty common thing for this model. Here's how I fixed mine: I found an old knob from a junker 60's vintage stereo preamp. (Possibly a Heathkit or similar) I was able to pry the gold insert from the preamp knob and glue it onto the CTC-11 knob. Perfect match to the other knobs!