21

The Williams 21, manufactured February 24, 1960 (model 233) and designed by Harry Mabs, is an electro-mechanical pinball machine built around a cards and gambling theme. Its backglass design closely resembles Williams' 1960 'Black Jack,' while its playfield layout more closely matches Williams' 1958 'Club House' (production notes indicate it was in fact built using Club House's tooling jigs).

Gameplay-wise, 21 features two flippers, three pop bumpers, two passive bumpers, two slingshots, three kick-out holes, two standup targets, and a gobble hole, providing numerous ways to score. Its standout mechanic is the scoring system based on the card game blackjack: the player's card score increases dynamically during play via bulb scoring and score reels, while the dealer's score is revealed on the backglass at the end of the game. Beating the dealer's score without going over 21 awards a replay. Sound effects consist of two bells and a knocker.

As a gambling-themed EM machine built on repurposed Club House tooling, the Williams 21 illustrates how manufacturers of the era adapted existing hardware to new themes.