Green’s Dictionary of Slang

a dice game based in bars (e.g. in Chicago and San Francisco) which could be played for a wager of 25 cents: if a player threw 26 in three attempts, they won $3.00 worth of drink; thus 26 girl, the (attractive) young woman who runs the dice game; 26 table the table at which a young woman offers to roll dice with any challenger; the game was seen (see cite 2016) as a possible front for sex work.

1944		Harper’s Mag. 189 48: Estelle Carey, nominally a 26 girl, had an income of at least $500 a week.
1949		N. Algren Man with the Golden Arm 313: They had let the 26-girls cheat them without a rumble.
1950		Lait & Mortimer Chicago Confidential 12: At the dice table the 26-girl sits in boredom. A glass of cheap mixed whiskey stands at her fat elbow.
1960		L. Mortimer Women Confidential 34: [Estelle Carey] worked for him as a ‘26-girl,’ managing a dice table in a cabaret he operated. Nick [Dean] was a big shot in the mob so his voluptuous babe didn’t drag down a measly $50 per like other girls, but cut herself $ 500 a week.
1995		(con. early 1950s) W.F. Roemer Accardo 109: Every ‘26 game’ had a ‘26 girl.’ [...] For a quarter the customer shook the dice in a leather cup, then rolled them out on the board, hoping that they would add up to the magic number of 26.
2008		(con. 1952) F. Holland Debut: Chicago, 1952 161: The sign in the front window of the Janus Door advertised for ‘Waitress and/ or 26 girl’ for the dice table.
2012		(con. 1947) J. Gusfield Deadly Valentines 196: By early 1947, Louise is employed at the King of Clubs, a gambling dive at 2839 Broadway. She is a dice-game attendant, a ‘26 Girl’.
2016		C. Di Edoardo (con. 1940s) Lanza’s Mob 186: [T]he Chicago variant was conducted by a ‘26 girl,’ whose job it was to schmooze the customers as well as run the game [...] some suspicious folk, like the Chicago Police department saw this as a front for sex work.
2017		(con. 1949) D. Travis Harold, the People’s Mayor [ebook] Harold [Washington] was in Anne's at the time a Black student, Mildred McNary, was told by the 26 girl that she could not play because she was on a list of people not allowed to play the game.