The lighter yellow books with chocolate brown binding are facsimiles of the original series.  The original series was considered by some to have had a higher literary content and possibly greater religious content.  The Revised series, with reddish brown bindings and blue swirls upon the cover, removed much of the original religious content and literary content to create a work that de-emphasized regional and other differences.  Another revision took place at the turn of the last century and possibly another circa 1920. Each revision significantly changes the content of the reading material.  I consider them all separate series, but am not going to separate them at this time.  The Speller, and 5th & 6th readers, were written by his brother.  Other clearly separate series are the Moore- McGuffey Readers, though I cannot specify particular differences.  Christian Liberty Press may have made a set in the past, labeling the books by letters.  I do not know if they are identical in content.  Now there are sets called "New" McGuffey's, which may or may not retain all original excerpts, but also include instructors content based on Charlotte Mason Techniques.