Here's a printers plate I cleaned up a little today and put some ink on it so
I could see what it said. I originally thought it was solid lead, but I
see it has a lead backing with a copper face.
It's a type of printing plate called an electrotype. After type is set, a wax cast is made of it, then coated with graphite and electroplated with copper. Once a thin copper shell is formed, it's backed with a lead-like alloy for added strength and durability. Sometimes, if the electrotype will be heavily used, the copper shell is nickel plated.
The plate appears to be an ad for Dr. J. H. McLean's Liver & Kidney Balm. McLean was a major manufacturer of patent medicines in the mid to late 1800's, and his St. Louis factory cranked out huge quantities not only of liver & kidney balm but also Dr. J.H. McLean's Celebrated Strengthening Cordial and Blood Purifier (85-100 proof alcohol!), Celebrated Catarrh Snuff, Volcanic Oil Liniment (crude petroleum), and Candy Vermifuge (anti-worm medicine).