Chevy Chevelle horn button is chrome and plastic and measures 3 7/16" across.  The face is in great condition with no scratches or discolorations.  My research tells me it is a horn button from a 1966 or 1967 Chevelle but what i can see of the part number does not match others I have found.  It looks like P-3878047 or P-3878017.  I can' be sure.  The back has two bent metal pieces for popping it into place.  They are tightly affixed.  I have had it displayed for years to remember my 1972 Chevelle Malibu I once owned.