“BYOB” (“bring your own bottle” or “bring your own beer”) and “BYOL” (“bring your own liquor”) are initials that date back to the Prohibition period in the United States. Restaurants would not provide liquor, but diners could bring their own.
   
“BYOL” is cited from 1919 and “BYOB from 1924. A 1919 “BYOL” citation references Prohibition regulations. A 1924 “BYOL” citation indicates that the term was used on ocean liners leaving New York City; under then-new rules on British ships, liquor would not be served by the ship until it was outside the twelve-mile jurisdiction of the United States, but passengers could bring their own (“BYOL”).
         
     
Merriam-Webster Online
Main Entry: BYOB
Function: abbreviation
bring your own beer; bring your own booze; bring your own bottle
       
(Oxford English Dictionary)
BYO U.S. and Austral., ‘bring your own’, designating a party, restaurant, etc. where food is provided but one takes one’s own drink; also BYOB U.S., ‘bring your own beer (booze, bottle, etc.)’.
1959 Amer. Speech XXXIV. 155 If they decide upon a party, they throw a ball or..in some cases, a *BYOB (bring your own bottle).
1975 New Yorker 26 May 32/1 Our parties are not just BYOB but also BYOW (Weed) and BYOBR (Brown Rice).
1984 M. FERMAGLISH Mollie’s Rules for Socially Inept iii. 72 As long as the invitation doesn’t say ‘BYOB’, they’ll show up.
         
15 June 1919, Des Moines (Iowa) Daily News, pg. 3:
Ashton Clemens has a new joke which he confessed is not original, but good.  Do you know what the new “P.S.” will be on formal invitations, instead of “R.S.V.P.,” after July 1, he asks.  His answer is “B.Y.O.L.,” which means, “bring your own liquor.” 
 
15 March 1924, Evening State Journal (Lincoln, NE), pg. 1, col. 1:
BYOL IS NEW SEA SIGN
Bring Your Own Liquor to Be
the Slogan of Liners, Since
Seizure of Orduna.
NEW YORK, March 15.—“BYOL” is going into the nomenclature of the big ocean liners alongside SOS.
 
That is to say, as a result of the recent seizure of the Royal Mail liner, Orduna, British steamship companies are passing the unofficial tip to traveling Americans to “Bring your own liquor” if they can’t wait until their boat passes the twelve-mile limit.
   
6 April 1924, New York (NY) Times, pg. 1:
“B.Y.O.B.” on Invitations
To Army Reunion Stirs Drys
WASHINGTON, April 5.—The mysterious legend “B.Y.O.B.”. inscribed on invitations to a dinner to be held in New York Monday night under the auspices of former army officers, stirred up something of a commotion today among in Prohibition circles here.
 
The Anti-Saloon League interpreted the inscription to mean “Bring Your Own Booze,” and Wayne B. Wheeler issued a formal statement calling on Congress to “oust” army officers who violate the liquor laws.
   
Secretary Weeks did not share the apprehensions of the dry leaders. he indicated that he regarded the four cabalistic letters as a jest, or perhaps as meaning “Bring Your Old Buddies.”
 
11 April 1926, New York (NY) Times, pg. 1.
“B.Y.O.L.” which means “bring your own liquor,” is concerned it is ancient history in the Provinces (Canada—ed.) in which the Government system works.
 
1 March 1929, New York (NY) Times, pg. 17:
Senator SIMPSON of Hudson County, who has a sharp wit and loses no opportunity to nag the Republicans, scored heavily when the program for the visit of a delegation of the New Jersey Legislature to Washington came up in the Senate at Trenton. It bore the “cryptic initials” B.Y.O.L. President MATHIS declared that they stood for “Bring your own luggage.” Thereupon the Hudson Senator interpreted the letters to mean “Bring your own liquor.”