Just wanted to say a thank you to the OP.
Whilst removing the steering wheel airbag on my car I managed to whack one of the stereo knobs with a ratchet and left a small mark on it.
From your photographs I was able to identify my stereo and search on the internet.
This led me to Pacparts.com in the USA.
The replacement knob was delivered today - $5 knob & $36 shipping but much cheaper than a replacement stereo. Gratifying to know you can still fix some things in today's throw away world.