Updated info: After removing the tach and opening the case, Tom you were correct, debris was stopping the needle. It now moves freely over the full range. I assume this means it really has been working most or at least part of the last 25 years. 🙂
The debris issue needs addressing, it seems the reflective paint inside the case is flaking off, pretty sure that is the source. Suggestions anyone? clear coat the existing paint? Can I get reflective paint to recoat? It also appear the black paint on the center of the needle is starting to flake.