Unless I'm mistaken, since the front bumper is not a 400 bumper, I would guess that it was in an accident at some point(why change bumpers). You'll want to check the frame, etc.
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto