Boston, New York, Chicago, Atlanta, San Francisco, Dallas: Allyn and Bacon circa 1950. A printing plate used to print a portion of the front cover of the book Adventures in Science with Bob and Don. Made of a curved piece of brass with the image to be printed raised and the balance removed. The "length" is 6 inches. The curve is approximately 2 3/8 inches high (measured off the table when placed flat). Width is 5 inches. Stamped in the plate is "0001" which we expect means that there were multiple plates - we've just ordered a copy of the book which has several colors stamped on the cover (see image). This plate would be used to print the darkest color, a nice dark blue, small remnants of which appear attached to the plate. Another plate would have been used to print another color. The plate has 5 holes for attaching to the printing press. Very Good. 
From the Rainbow Series "Orange".