Yes, your lack of the volume control knob must be highly irritating, the rest of the Concert looks mint.
I suspect the Chorus head unit has the same volume control knob, but I have not got one to compare (comment anybody). If this is true there is an option on EBay, not cheap depends how irritated you are!, but that's the going rate.
YOUR RADIO MUST LOOK LIKE THE ONE IN PICTURE #4.
www.ebay.co.uk
The knobs just push on, (and pull off which is the root of the problem). Be careful to not press too hard or the knob will overly depress (common problem) and you will have to pull off and reassemble (fiddly) the three components and try again.
Climate refurb next!?
Andy