Saw this 914-6 tach for sale. Interesting part is that it has the early 911 posts for clamp to dashboard. If you look a some of the early 914-6 tach pictures you will see the posts shown in service manuals. I have never seen on a -6 though. This one shows a April 1971 build date. Just an anomaly ?

Bezel completely undisturbed? No possibility of 914/6 guts in a 911 can homebrew?

If all factory sealed, probably an mis-build / quality issue. I hear there was lots of dope smoking in the 70's

Using left over cases is my thought.