VGC Stettler, never digitized to my knowledge.
(Thanks Mark Simonson for the original identification)

By Wayne J. Stettler, 1965 for VGC. There is a font out there called GT-Stetler (yes, one t) that is only the caps of Stettler. Not much help, but that is the only digitization I'm aware of.

If you are looking for substitutes you could use Identifont for Optima http://www.identifont.com/find?font=optima&q=Go and go through the similar fonts.
Intersting ones include Cyrus Highsmith's Amira, Thierry Puyfoulhoux's Alinea Incise, José Mendoza y Almeida's Pascal, Gábor Kóthay's Minerva Modern, and Hermann Zapf's Optima aka Classico or Zapf Humanist 601.
Don

Cheers guys, fantastic!

Corrected post:
Good find Mike. I was curious about what the "GT" refers to. It seems to be for Gemini Fonts
Another foundry that uses prefix _GT_ for its fonts is Grilli Type but they seem to have no relation to Gemini Fonts. BTW I have seen a version labeled GT-Stettler. Poor quality digitization, and as Mike said, caps only.
Don

Myfonts page on the designer, Wayne J. Stettler http://www.myfonts.com/person/Wayne_Stettler/
Don