What is the construction/material at the base of the storage container, on the inside bottom. Mine has a rust-like look and slightly fragmented piece. Does one remove it and replace with a new bottom liner? For now I've just added a thin piece of cotton as a barrier. Thanks

There is a lot of useful information here

such as this: red restoration



The rubber material can deteriorate over the decades, which in some cases is not good for the Curta... Instead of felt, any
chemically stable material can be used, such as silicone.