Main Stemmer (Mainstemmer)

“Main stem” was a popular nickname for Broadway, especially in the 1920s. A person who worked on the Broadway or who enjoyed Broadway shows was a “Main Stemmer” (or “mainstemmer”). “Broadwayite” is a similar term.
 
“Main Stemmer” was cited in Variety (New York, NY) on March 20, 1920, and “main-stemmers” was cited in the Daily News (New York, NY) on December 6, 1927.  “Main Stem” and “Main Stemmer” were only infrequently used after 1940 and are of historical interest today.
 
   
19 April 1915, Pittsburgh (PA) Post, “‘Unsuccessful Crook’ Wants To Be Straight,” pg. 7, col. 7:
“I’m known as ‘Hoosier Slim,’ the Main Stemmer.”
 
20 March 1920, Variety (New York, NY), “Moving Pictures: Strand” by Fred, pg. 53, col. 1:
If their stuff doesn’t hand a laugh to the hinterlanders it is only because their education has been neglected, but the Main Stemmers will get many a laugh out of the titles in “In Search of a Sinner.”
 
6 December 1927, Daily News (New York, NY), “Six-Day Sidelights” by Sam Schnitzer, pg. 58, col. 2:
Mrs. Lillian Dixon and other celebrated main-stemmers can’t seem to keep away from the panic.
   
5 July 1928, Life magazine, “Along the Main Stem” by Walter Winchell, pg. 6, cols. 1-2:
Her converts included ham actors, good actors, managers, producers, song-writers, dope fiends, pickpockets and other Main Stemmers, and then she left us all flat, for love.
 
28 October 1928, Daily News (New York, NY), “Kid CHocolate Does His Stuff in St. Nick’s Ring” by Jack Farrell, pg. 84, col. 1:
St. Nick’s rink, the Monday night “hangout” for main stemmers, will feature Kid Chocolate, the dynamic little Cuban, and Joey Ross of Hoboken in the main bout of ten rounds.
 
3 November 1928, Emporia (KS) Daily Gazette, “Your Broadway and Mine” by Walter Winchell, pg. 4, cols. 6-7:
Mainstemmers are wagering that Alexander Woollcott rejoins The Sun as dramatic critic when Gilbert Gabriel switches to South street.
 
19 November 1928, Evansville (IN) Courier, “Your Broadway and Mine” by Walter Winchell, pg. 6, col. 6:
“He was the ‘heart’ of the racket,” said one Mainstemmer.
       
Google Books
The Pig Is Fat
By Lawrance M. Maynard
New York, NY: Farrar & Rinehart
1930
Pg. 166:
With Benny in her thoughts she had written the song that sentiment-loving Main Stemmers cried for — “I’ll Still Be Your Pal If You Need Me.”
 
23 November 1935, Detroit (MI) Evening Times, “On Broadway” by Walter Winchell, pg. 7, col. 2:
Broadway Dictionary
(...)
Main Stemmer: One who tries to figure everybody out—but himself.
   
Google Books
Murder Steals the Show
By Leon David Hirsch
New York, NY: F. Fell
1946
Pg. 100:
Next night’s rehearsal was delayed an hour by Charlie Finnglanz being detained over on the main stem because of a conference with important Broadway main stemmers.
 
17 June 1965, San Diego (CA) Union, “Walter Winchell’s America,” pg. A44, col. 1:
Main Stemmers generally congregate at well-known eateries.
 
22 March 1968, St. Albans (VT) Messenger, “Everywhere” by Walter Winchell, pg. 4, col. 6:
Vignette: Many Main Stemmers always knew when it was 5:45 sun-down-time.