Tail Light / Side Marker Light. 
Part # SAE P1A74

GM Automotive
Part # 912667 - 5987999 - 5944550 A - 5944600 R - 5967999 - 5936635

For use with 
1971 - Chevy Vega Side Marker Light 
1977 - 1979 Chevrolet Chevette Side Marker Light 
1977 - 1979 Buick Opel Side Marker Light 
1977 - 1979 Pontiac T1000 Side Marker Light 
And possibly other small / compact cars manufactured by General Motors in the 1970's / early 1980's 

These tail lights are New, Old Stock, Purchased in a surplus buyout.