FWIW, they were available as an option on any 78 280z maybe other years. I've got the original dealer brochure showing them on a light gold like mine as well.
I'm looking for a working tach for my 280z. I believe the 75-76 versions have a different font for the numbers and lettering, so I'm primarily interested in a 77-78. I'm sure someone out there has swapped gauges and has one laying around. Thanks!