[Jeffhs]

At least one other manufacturer in the '60s had a radio with two series strings--Zenith. My MJ-1035, from 1965, has two strings as well, with a filament transformer feeding both. My set has eleven tubes (two of which are 50EH5s), with one 50EH5 in each string.

The 17C9 is a rare bird, indeed. The last time I saw a 17C9 was in an article in Popular Electronics magazine from the early 1960s. I had never seen a ten-pin tube with the tenth pin in the center of the base before then, and never saw one after that until now. You may have problems finding replacements if you ever need to change it, as I don't think they were all that popular when new and may be very difficult to find these days.