[Arran]

It's likely a metallic paint, before attempting to clean make sure that the paint is till adhered to the glass. If it's well adhered and not bubbled or flaking I would test an inconspicuous corner of the printed side with a solution of soap and water before attempting to clean the rest. The front side of the glass is perfectly safe to try cleaning. No matter what you use I don't think that stain will come out, that stain is some form of oxidation of the metallic paint and I don't know of any fix for it. If it really bugs you, and you can't live with it, you could consult Radio Daze about making a reproduction which is a service they now offer. It may be simpler and cheaper to just buy a better glass salvaged from a junked post war Philco model with the same dial rather then getting a reproduction made, in general post war floor models are not that sought after.
Regards
Arran