1909 - Edwin Frazier's early design had a big plus since it was a simple external mix. It used a round "carousel" design for the paint cups. Each color had it's own paint path which eliminates the chance of accidental color mixing. To change the color you simply rotate the cups until the desired color is in front of the air jet.

The big disadvantages are that the simple 90 degree external mix gives poor atomization and it's large and cumbersome to operate. It was probably pretty handy around rural America as you could load up paint, pesticide or whatever and simply twist to spray the fluid that was needed at the time.