I am sure that the frame was designed to take a piece of glass

Often the original gets broken, or separated during moves, etc., and another item is used to replace it. And yes, it should be a good fit, otherwise it would be very obvious. I have known dealers buy frames specifically to upgrade them by finding an item that would make the piece 'whole' again, so getting the required uplift on the sale price. It's been done for many years, which is why I habitually question framed pieces

It works sufficiently, so, happy days

Nigel