According to the "75 years of TV/ TV history site", 1951 appears to be the transition year for Zenith... they made roundies in 12", 16" and 19" and early rectangulars in 17" (perhaps it was a 16"?) and 20". To my eye, this set looks earlier.
By 1952 they appear to have switched over to the "standard" 17" and 21" sizes that dominated the mid 50s, and their sets looked pretty much like everybody else's.
http://www.tvhistory.tv/1950-59-ZENITH.htm
I have not seen a '51 or earlier Zenith rectangular before.
OT: Einar72... Is that an Abyssinian cat?
jr