Mike,
I have a new in box 'Datcon' tach and a new in box mount for same tach.
The tach is 31/2" diam and four (4) thousand rpm. MADE IN USA Datcon part number 101853. Haven't checked on year of manufacture on printed material in box, but think it is 20/30 years old.
Send me pm if interested and I'll send you some pics. I'd like to see pics of what it is that you've got also. Rick