It turns out that these "badass" ladies were originally holding up signs spelling "WAMPAS 28," as in the Western Association of Motion Picture Advertisers (WAMPAS) Baby Stars of 1928.
Since 1922, the all-male publicists from various movie studios who made up WAMPAS had tapped 13 bees knees actresses they thought producers should pay attention to, providing easy fodder for the emerging Hollywood media machine (see also: Vanity Fair's annual Young Hollywood issue). Better yet for the publicists, the annual "Baby Star" awards doubly served as a strategic publicity stunt for WAMPAS members.
Each year through 1934, WAMPAS selected 13 Baby Stars -- although they bumped it up to 15 in 1932 -- and in 1935, the publicist interest group folded along with the campaign. A few of the Baby Stars would grow up to became Hollywood legends: Clara Bow, 1924 Baby Star; Joan Crawford, 1926 Baby Star; and Ginger Rogers, 1932 Baby Star.
But now thanks to Photoshop, they're probably now best known to pop culture as the black-and-white "Badass" ladies, which isn't such a bad legacy either.