1920s - 30s? LOL - only off by 30 - 40 years... Plus, TV didn't really become available until 1939 or so. And $700? This person is smoking serious loco weed.
The service tag has a zipcode on it. The USPS didn't start using zipcodes until 1965. Before that it was zone codes like Chicago 50, Illiois. To me this TV looks very mid 60's.
I tend to think that sellers who think something they have is much, much older that it actually is because they themselves are very young which in turn makes me feel very, very old which I guess I am.