This applies to black clocks (not ivory). First, the dials were plain inside the numerals. Later, they had a printed square inside the numerals.
Dial 1
Dial 1
Dial No Square (black clock):
The dial is plain inside the numerals. Used through ca. 1935.
Dial 2
Dial 2
Dial Has Square (black clock):
Inside the numerals are three concentric squares closely spaced with rounded corners. "Westclox" replaces the three lines in part of the bottom of the square. Used starting ca. 1935.
