I agree that the Viper was the successor to the 4" cobra and an alloy frame with ejector rod shroud, but though the grips were the same style as the DS of that era, were much longer. Are you sure the Police Positive lookalike of this era didn't have a steel frame? I think they finally dropped the Special ( ie PPS vs PP )designation which had up to that time designated the longer frame and cylinder to distinguish it from earlier models which could only chamber the 38 S&W/38 New Police.
Maybe you're right, but I sure wouldn't have thunk it.