“31” or “Thirty-one” (lemonade)

 
 
Newspapers.com
10 June 1929, Pittsburgh (PA) Post-Gazette, “Pittsburghesque” by Charles F. Danver, pg. 8, col. 6:
The Soda Jerkers’ Code.
The young bartenders in one of those big soda emporiums downtown have a secret code. They use it in relaying orders behind the bar.
(...)
“Thirty-one!” (Lemonade.)
 
Newspapers.com
9 February 1930, Tampa (FL) Sunday Tribune, “Soda Jerkers Here Talk in Language Hard to Understand,” pt. 4, pg. 6, col. 2:
31—Lemonade.
 
Newspapers.com
10 July 1985, The Times-Mail (Bedford, IN), “Soda jerk unique on American scene” by Ink Mendelsohn, pg. 5, col. 5:
THIRTY-ONE—lemonade