Martin Needleman outed his eBay Zunow
In the entertaining and accurate auction description, he describes the various decals and what they mean, all but one: "There is also a decal which reads 'Zunow Horn System' and I haven't the foggiest idea what that means."
Horn System refers to the special seat tube. It is normal 28.6mm (1-1/8") over most of its length, except just below the front derailleur it flares or "tapers up" to 31.7mm (1-1/4"). Actually that's the dimension it would be if it were left round - it is ovalized such that the front-to-back dimension stays about 1-1/8", increasing the side-to-side dimension to about 35mm (1-3/8").
This is an excellent design, which I have used on lots of frames. (Used to be, any framebuilder could buy this tube from Tange - don't know if it's still made.) On most any normal diamond frame, (i.e. not counting Rigi, Thanet and other "funny" bikes), the sides of the seat tube at the BB shell see a very large stress, as evidenced by the frequency of fatigue failures there, and the flex that causes the noticeable chain rub on the F. der. cage. So adding the minimal extra weight of this flare is grams well-spent.
Other brands (notably Serotta) took it further, making the large part of the seat tube larger still, and running the large part farther up the tube, but this adds more weight and prevents using normal clamp-on front derailleurs (talking period equipment here). Also I think it is out of balance, if the other tubes are normal - I like stiffening up the seat tube just enough to bring its stresses more or less to the same levels the rest of the frame. My gut reaction is the Horn System is a balanced design with few drawbacks.
Seattle WA USA