AL847) ?Vintage railwayana/optical glass? Round glass 6.25" diameter 0.5" thick

Detailed Condtion : This came in a box with railwayana and optical bits but I have no idea what it is - too big for a lamp glass? Not a lens though? See pics. There's wear and tear - looks like it was remove from something - mesh on the back. Rough on the back.

This sale is for a round piece of thick glass measuring 6.25" diameter.

Definitely looks like it has been taken out of something - but what? It came with a whole host of collectable items - some railwayana and optical lenses - so I'm assuming it's one or the other but I could be wrong. It measures just over 0.5" thick and is in a worn condition. There's mesh on the back of it (see pics). Any info gratefully received.

Times are tight I know so, thought it would be good to remind you that I do accept weekly payments on all orders and then send the item when it's paid up. Just get in touch after you've bought the item to arrange it. No interest charged, of course. Just pay the actual buying price + p&p as you usually would.