I have one of the original 1966 issued AMT model kits of the 66 Riviera and it shows a tach located in the console, where the last woodgrained piece is on the real cars. Since AMT had to make the models based on early Buick blueprints, it's a good bet that Buick seriously considered installing a tach there. It wouldn't have been much use, because it lays almost flat. I'll try and find the model kit and see if the tach shows in a pic.
