...what he said.
My bet is that its a tach made by RAC or some other "second rate" company, and sold through a JC Whitney type catalog, hence the lack of logos or names on the tach itself. Dixco used to to this as well, and sold hood tachs, gauges, etc. through JC Whitney, with no logos or brand name present. I don't think this is a Dixco tach though, as it doesn't look like any I've seen with the Dixco logo on them.