What I meant, is of the 3 FM radios Zenith released for 1931, you find the "Spinet" quite a bit more than the other two.
Right now, I have 3 of the 10H571 Spinet's in my collection. It took me a bit of searching, but I finally landed a 10H573 standard style version of that radio. You just don't see them very often. let alone the Chairside 10H551!
It must have been popular with the public as they can be found.
According to the Zenith book (Zenith The Glory Years, 1936-1945 Illustrated Catalog & Database) the 10H571 has a rarity of "Somewhat Uncommon" whereas the 10H573 and 10H551 are rated as "Uncommon to Rare"
The value of the 10H571 Spinet is certainly more than the 10H573 or the 10H551 Chairside as it is a very unique piece.
As is the case with many radios. Rarer doesn't always = more valuable.
Oh, The name "Microstatic Frequency Modulation" was just a name Zenith used. Zenith President E. F. McDonald was a full supporter of E. H. Armstrong. They definitely had an Armstrong license.
RCA had absolutely NO connection to Armstrong at that point and seemed to be at war with him as RCA President David Sarnoff was pushing Television as hard as he could! He wanted FM squashed!
RCA never had an Armstrong license and did everything they could to keep Armstrong from profiting from FM. Eventually driving him to suicide...
It was a sad story.