Raided the Modern Mechanix site to see what advertising media were budding in the 1930s.
1931: Someone had a great idea to put ads in the middle of Newton color disks (like these). Notice the combination of "high school science classes" and "package of cigarettes".
1933: Sales robots! Some guy hid phonographs behind cut-out cardboard dummies. Guess nobody could tell the difference anyway.
1934: Wow, something like this passes for emerging media even today. Person standing in front of a mirror triggers light that illuminates an ad layer behind the mirror's surface.
1936: Advertising mirror, perfected. Now with a phonograph record triggered by a light sensor.