Yes, there is, but unfortunately it has not been digitized. There is also a bound edition of the 1919 Graded Readings with the key (titled “Gregg Writer Graded Readings”) that was specially published in 1925 for those teachers who used the Gregg Writer magazine in their classes “when 90% or more of the pupils are subscribing.” It was not available for purchase due to the high cost of production: cloth binding stamped with gold, and with gold edges, according to the announcement published in the October 1925 edition of The American Shorthand Teacher. As with the key, this book has not been digitized either.

Worldcat lists several libraries with copies of the 1919 key and the Gregg Writer Graded Readings, such as the NY Public Library (which holds the John Robert Gregg Collection), the Rider University Library (holding the Louis A. Leslie Collection of Historical Shorthand Materials), and the Library of Congress. The NYPL has a digitizing service for researchers — you may request materials to be digitized for a nominal fee depending on the number of pages. I have used the services of the NYPL remotely and also have visited the LOC to have materials photocopied and scanned. You may want to explore that option if you want to get those books.

Lastly, you can always ask here if you have any doubts about outlines.