This is a wood ad mat for the Superman comic strip dated July 7, 1951. In the days before computerized page layout and electronic typesetting, wood ad mats were molds used to create lead castings for letterpress printing. Hot lead was poured into the wood mat to create the casting. When it cooled off, it was secured to the press and inked, and paper was put on top, then brought down under pressure to create the print.
Typically, wood ad mats were thrown out after the cast was made, so they’re pretty rare, but I don’t know that they’re particularly valuable because they are so esoteric… not exactly an exciting display piece, but it’s an interesting piece of printing history from the pre-digital world.
The ad mat is made of a wood pulp product… the closest modern approximation of the material to my uneducated eye in terms of composition and texture would be non-styrofoam egg cartons.
If you’d like to purchase a wood ad mat for yourself, keep an eye on this eBay seller’s auctions.