Welcome to the forum!
I've never heard of Zenith for a brass instrument before. But Whittle Music Company existed during the 20th century, although I'm seeing conflicting reports of just when they did business.
I suspect Zenith is a name they chose to have stenciled onto brass instruments, which I assume were made by a standard American company.
Perhaps if you give us more photos of the horn some of our members can make a better guess.
Dave Werden (ASCAP)
Euphonium Soloist, U.S. Coast Guard Band, retired
Adams Artist (Adams E3)
Alliance Mouthpiece (DC3)
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Sorry, instrument is 150 miles away and this is the only photo I have. I also think Zenith was the name they chose and they were probably made by Conn, King, Etc.
I don't know anything about it but Dave's thoughts about stencil instruments sound right on. I plugged it into google and found this:
http://www.brasshistory.net/Whittle%20History.pdf
Which suggests they were made by Pan American (which was a student line of Conn) and then I found this serial number list for them which seems consistent with the Zenith serial numbers on the other page:
https://cderksen.home.xs4all.nl/Conn...anAmBrass.html
So it seems like your friend's instrument would have been made in 1929.
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Barry