50 or so years ago the metal capped OC71 transistor used to work well as a photo-transistor if you removed the black outer coating.
In effect it then became the much more expensive OCP71. At that time the glass encapsulation was filled with what looked like a clear silicon grease.
It didn't take long for the manufacturer to twig onto this trick and started filling the OC71 bodies with a translucent filler which obscured the junction, so ensuring sales of their OCP71