“10” or “Ten” (root beer)

 
 
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.
(...)
“Ten!” (Root beer.)