Harper & Brothers (publishers) used a two letter system from around 1912 to indicate which printing a particular book was printed. The letters H-A refer to when the particular book you have in your hands was published. H refers to the month of August. A is possibly 1926. In other words, the book you have was likely published in August of 1926 which may or may not match the copyright date.
Re: I have a 1921 copyrite of "Howard Pyle's Book Of ...
Harper & Brothers (publishers) used a two letter system from around 1912 to indicate which printing a particular book was printed. The letters H-A refer to when the particular book you have in your hands was published. H refers to the month of August. A is possibly 1926. In other words, the book you have was likely published in August of 1926 which may or may not match the copyright date.