“51” or “Fifty-one” (hot chocolate)

 
 
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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.
(...)
“Fifty-one!” (Hot chocolate.)
 
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9 February 1930, Tampa (FL) Sunday Tribune, “Soda Jerkers Here Talk in Language Hard to Understand,” pt. 4, pg. 6, col. 2:
51.—Hot chocolate.
 
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25 May 1934, Daily News (New York, NY), “Mainly About Manhattan” by John Chapman, pg. 60, col. 3:
Add soda squirt slang, from Tommy McCord.—One limey,,,a toasted English muffin. Hold the hail...no ice. Eighty-four...a customer who tips. Eighty-six... one who doesn’t…Fifty-one...hot chocolate. Eighty-one...a glass of water. Freeze one...frosted chocolate.
 
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8 April 1939, Sheboygan (WI) Press, “What Is Your Number?,” pg. 14, col. 2:
51—Hot chocolate. 52—Two hot chocolates.
 
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1 April 1946, Roanoke (VA) World-News, “Observant Citizen” by W. C. Stouffer, pg. 6, col. 7:
“Fifty-one” means a hot chocolate and “ninety-one” for an English muffin.
 
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27 September 1981, The Herald Statesman (Yonkers, NY), “Counterwailing” by Dan Carlinsky, Suburdia Today, pg. 18, col. 4:
51. Hot chocolate. (52 = two hot chocolates, etc.)
 
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29 November 1997, Pueblo (CO) Chieftain, “Word Mysteries” by Chris Woodka, pg. 5A, col. 4:
51—Hot chocolate, with 52 meaning two cups, 53 three cups, etc.