I've been doing a bit of research on this subject, since just acquiring a legit red cab (TKG2). There is a great thread, albeit extremely long, with lots of good information at KLOV:
https://forums.arcade-museum.com/showthread.php?t=64571I've made it through about 50 pages of that thread, and never once have seen any mention of a wood knob. This is also the first I'm hearing of it in general. Not that I'm an expert by any means, but I have been devoting some time to the subject recently, and I can say with 99% certainty that the wood ball top was not original to DK cabs, red, blue or otherwise.
Some more info dedicated to Radarscope here:
http://www.ostermayer.ch/kontakt.html. Looking at all those pics, the ball tops are all the standard black. I'm leaning toward 99.9% certainty on the wood ball top being aftermarket.