Menu Protectors

I need some information and hope someone can help me. During the 1930s, possibly earlier, small diners/cafe's/restaurants may have had their menus in a clear protector, usually with some metal edging on the corners. Sometimes these were single pages, sometimes multi-page. I know these are still made today of vinyl ( the image I'm using I found on the internet from a modern day company) but what were they made of during the pre-WW2 era? Celluloid? Some early type of plastic? I am trying to find the exact/correct term for the era.