Until now I had never seen a CRT that small. Where would a tiny tube like that be used, anyway?
Too small to be a television CRT. I once saw a TV (in an old issue of the defunct Popular Electronics magazine, IIRC) with a 3" tube built for a doll house, but this little button-shaped (and sized) one is a new one on me. Probably one of a kind.
BTW, in answer to the post from the person who has the pink portable TV and who said "can you imagine getting a TV like this in 1970 (or whenever you were 10)": I was ten years old in 1966. I don't think that particular TV had even been thought of until the 1970s.
My first TV was, IIRC, a 16" 1950s console Capehart set, followed, again IIRC, by a 17" 1955 Emerson metal-cased portable. The smallest b&w TV I ever had was a 12" 1969 Sharp portable that lasted only three years (tuner problems).
I had several small b&w TVs after that, with the smallest color set (after my Silvertone CTC-12 clone bit the dust in the early '70s) being a 16" Sears-branded (but probably made by Toshiba) metal-cased table model, a trash find in 1975. The smallest color set I have ever seen was in an eBay posting a few weeks ago; the set was a Sony Trinitron 6" TV. I bet that set would make excellent HD pictures, with a screen that small.