Im pretty new to this… I recently aquired a small rectangular silver metal dish. It measures approx 25cm x 19cm and is fairly heavy. It has some markings on the back…I can make out the May & Co bit … but Im not sure about the rest. It looks like and S with a bracket at the top and bottom of the letter. The remaining letters could well be EPNS or even FPAS … I also discovered by chance that there are two parts to this. It looks like an inner and outer dish. Only the outer dish has markings though.
EPNS (if that is what it says) stands for electroplated nickel silver; i.e. silver plated onto a base metal. As this is normally never date marked it will most likely prove very difficult to give an accurate date. Any sort of dating is impossible without pictures of your dish and especially of the marks on it.