The date on Ethel's grave was January 13, 1933, which is 1-13-1933, so you want to set the numbers on the box to 1131933.

Any way that you do this is the right one, but the fastest way I've found is to turn the left knob first, then the right, and just keep alternating them until the puzzle is solved. So that's Left - Right - Left - Right - Left - Right - Left - Right.