Green’s Dictionary of Slang

a fraudulent ‘gambling’ game, typically played at a racetrack or a carnival midway; thus under-and-over merchant, the proprietor of such a game.

1894		A.A. MacInnes Straight as a Line 133: [W]hat a day the men who played games of chance on the racecourse were sure to have! —the ‘three card’ men; ‘under and over’ dice manipulators; ‘Who'll have the lucky number seven?’ ‘marble men’; ‘spinning jenny’ spielers [...] and a host of others.
1899		W.H. Brown On the South African Frontier 407: [T]here [ie.e at a racetrack] were echoed the voice of the man at the fortune-wheel, the proprietor of the under-and-over table, the bookmaker, the purveyor of refreshments [etc].
1937		C. Drew ‘The Chameleon’ in Bulletin 3 Novt. 4/1: [T]here was some sharpshooters on the flat. There was blokes who ran blackboard totes, monkey-sweep men, tip-sellers, under-and-over merchants, three-board artists and Yankee-sweat men, all combined in a frontal attack on the geese with the golden eggs.
1946		Billboard 9 Mar. 57/1: [advert] baker’s game shop [...] Under and Over Cloths / Beat the Dealer Cloths / Jumbo Skillos.
1951		Billboard 1 Sept. 62/4: last call / FOR MISSOURI FAIRS / want want / Agent for Count Stores, Skillo, Pan Game, Under and Over [etc].
1970		(con. mid-19C) K. Chesney Anti-Society 230: [An] under-and-over man inviting passers to drop stakes on his patterned sheet.