I got locally at a reasonable price a set including the IIIc with 50mm Xenon, the Schneider 80mm and 35mm in their bubbles, and the auxiliary viewfinder.
The chrome in the top of the body was long gone, so the technician took it out. I can brag that this is a "special-series" yellow-top IIIc.
The Xenon is a fabulous lens. This was at noon, to check RF focus at close distances.
I was curious to see how the auxiliary lenses performed.
Despite the help obtained in another thread, I found the lenses difficult to focus because of the complex procedure to follow. The 80mm is especially clumsy because of its size and lower DOF. When I got it right (not often), the results were quite acceptable, although not on par with modern equivalents. An example of the 80mm Longar:
The tiny 35mm Curtagon is easier to use, and the shorter focal distance is more forgiving of operator confusion.
I do not think that I will be using these auxiliary lenses a lot, although they do their job. Great Yellow Father may have got its goal of an interchangeable-lens folding Retina, but I'd rather use the IIIS.
In fact, I have never heard of any other folder with interchangeable lenses. A dead-end of camera evolution, and maybe interesting for collectors just for that reason.
Thanks for looking and for any comments. Film was Pan F+ in Beutler.