Developing 60 years old Kodak Super XX - darkening after 24hrs

Viewed against a dark background in strong light, Super XX will typically exhibit a reflected positive image on the backside. This has nothing to do with silver being "redistributed". It occurs with many films to a degree, but most prominently with old thick emulsion ones pyro developed. Viewing developed films in this manner is often a helpful way to assess shadow content which otherwise is not evident viewed over a bright lightbox.