From 1920 through 1960, the English keys for the shorthand plates in the Gregg Writer and Today’s Secretary only appeared in The American Shorthand Teacher (1920-1950) and Business Education World (1950-1990).
Scans of every issue of The American Shorthand Teacher and Business Education World are available as separate magazine issues on the Internet Archive. (They were uploaded in April 2020.)
—Business Education World ran from 1950 through 1990. However, the shorthand plate keys disappeared from that magazine starting in September 1960, Volume 41. (I am not sure whether they reappeared in any later volumes.)
—You can download PDF files for separate issues of these magazines for 1920-1961. (The 1925-1961 issues are in the public domain because the copyrights were not renewed.)
—You cannot download PDF files for issues from 1962-1990. You can only check them out for one hour and read them online.
The Internet Archive homepage link for The American Shorthand Teacher and Business Education World (1920-1990) is below. The best way to find particular issues is to filter by year on this page.