No, that's a factory option. My sales literature for Firebird in 1975 shows it. That was the first year for it -- dunno if it was the last or not. In 1975 the big push was on for fuel economy and this was one of GMs responses. It showed manifold vacuum, the idea being that "green" denoted high manifold vacuum and thus good fuel economy, while "red" indicated low manifold vacuum because your foot was in the accelerator and your mileage was suffering. Dodge had something similar that caused lights on the top of the front fenders to illuminate when mileage was "optimal", IIRC.
