I have a Mystery Control as well as one of those chrome microphones, which I picked up separately for a few bucks apiece at different swap meets. I'm not crazy about the styling of the radios that use the Mystery Control, so I have passed on more than one "control free" set that I ran across here and there.
I think the Controls were often separated from the radios over the years. I have gotten more than one email from someone asking me to identify a "little Philco radio with a telephone dial" that they found in a shop.
The control is interesting, but kind of a gimmick in my view, judged by the fact that other manufacturers didn't rush to copy it somehow. There are other examples in radio history.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html