Hi Breezer,
I am a fellow Zeta owner, I think the arm is great; too bad I'm one of few that had that opinion. I chose it over the Alphason S shaped arm that was all the rage in the mid 80's.
I purchased it from Reference Monitor International, Carlsbad, Ca. March, 18th, 1986. I have the original box, invoice,and specifications. There is no serial number on the arm. I used a mirror to check on the bottom of the arm block but saw nothing. I plan on installing it on a SOTA Millenium table I bought at the CES. If I find one someplace, I'll let you know.
regards,Ken Fritz