Found it! I've found this information by typing in 'Pontiac Firebird design 1969', put it on pictures and 'grey'. It's translated from Dutch ...
The first generation Firebird needed an upgrade for 1970-1971. At least, that's what they thought at GM. So they designed a bigger and more luxurios model for the following year. But then the government decided it needed bigger bumpers, so the project was abandoned ... It was designed by John M. Mellberg, the exterior designer for Pontiac at the time.
I also found a few more pictures ...