I have also wondered about why they were called Radio nibs, but never pursued the question with the experts. I would bet that
John Gwin could tell you, or could recommend someone to ask.
There were a bunch of Radio nibs. They all (or all that I have seen) seem hold a lot of ink (relative to other dip nibs). They differ in their physical characteristics (shape, flexibility, finish, etc.) though, so it's hard to see any other common factor. Maybe "Radio" was the "e-" of its day -- a handy marketing tag to attract buyer attention, differentiate the pens from the competitors. Generally I like the Radio nibs for informal scripts, but for formal scripts I'd rather use a Gillot. If I were using a dip pen for everyday, I'd pick an Esterbrook Jackson Stub.
Here's
a good source for Esterbrook nibs. It amazes me that these were last manufactured so many years ago, yet there are enough around that you can still get them for a buck. How many millions (billions?) must Esterbrook have made?!