P.F. Collier & Son printed the 50th edition (that is, different set) of The Harvard Classics in 1956. Owners and prospective buyers of The Harvard Classics editions are often interested in the printing year of a particular edition. As mentioned before, not even the first editions were fully printed in 1909. First editions were printed in 1909 and 1910, and all subsequent editions were printed in 1910 or later. A printer's key could be used to describe the print run, but these were not used in the U.S. until the middle of the twentieth century. Copyright dates for book reprints are unlikely to identify the year of printing excepts for first four editions. For The Harvard Classics series, copyright pages of The Harvard Classics have no information about the printing year (or run) until 1956 when the publisher began including information about the year of the print run.