4x6 Glass: New old stock. These were for a Kodak professional film enlarger from the 1940s. It does not appear to be anti-Newton glass though. Just optically clear plate glass. It's 1.6mm thick. Good for picture frame replacement I suppose (it's almost exactly 4x6 inches - 101mm x 152mm). It's a bit foggy looking when I opened one (see the 4th pic) but cleaned off easily with glass cleaner.