"Gregg Shorthand in Three Parts"

Hi, all.
I just purchased an interesting little 3-volume set called "Gregg Shorthand in Three Parts."  It is the 1916 pre-anniversary version, and apparently it was issued in this 3-part format for some reason.  Part One is in a red binding, Part Two is tan, and Part Three is green.  The text is identical to the usual one-volume 1916 edition, even to the page numbers--the Part Two book, for instance, starts with page 46.  They are extremely thin little volumes.
Anyone know anything about this format, or why Gregg would have published it this way?  The three volumes, by the way, are in pristine condition--someone (the name in the books is Pauline J. Woolley, Riverside Hotel, Reno, Nevada) has had them carefully tucked away over the years.  It looks like Pauline never really used them much, if at all.

(by alex for everyone)