One New Old Stock Chevrolet Part #7803820, Power Steering Pump Reservoir Cap, is for:

Most 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 Chevrolet. (except Vega and Chevette) 

There is a bit of surface on the spring under the cap due to long storage.