I recognize these. I've made them. It is a copper printing plate. They are screened for use directly onto the paper, probably in a letterpress since they are flat.
They are made by exposing a print (copy) in a large process camera with a vacuum back to hold the film perfectly flat during a long exposure. The resulting very dense high contrast "lithographic" film is then contact printed onto a sensitized copper plate (gum bichromate process). The plate is put into an acid etch bath that eat away the exposed portions and leaves the unexposed portions (a dot screen) in a relief. Ink sits on the high spots that hit the paper when an inked plate (That's what you're looking at) and paper are pressed together in a printing press.
There's more but you'll find everything on the Internet. If not, I can answer any question you may have. I've done it and taught it. I still have a press in my living room that would accept it and make a nice print from it. :)
Edit: I see there're tacks on the edges. They hold the thin chamfered etched copper plate onto a piece of wood - probably plywood - to raise the surface to the international standard type height of 0.918 inches.
Cleaning is easy, wipe it with a slightly oily (keroscene) rag to soften and remove any ink hardened in the surface although it looks well cared-for by the last printer's devil (assistant). The rich patina of the aged copper can be removed to make it sparkle as new; but, that will remove some of the very detailed etched surface of the plate. It is best cleaned not polished to maintain its original integrity. Store flat or on edge. Caution: Copper is a soft, easily-scratched metal. Once scratched, cannot be repaired. For durability, copper plates were chrome-plated.
This would've made a nice cross post with the graphic design group, too.