I found this on a clock and watch forum which more-or-less confirms Andrew's flattening theory:
"The process was called "Planieren" (planing, grading) and was used to roughen the plates
and relieve tension. Junghans applied for a patent in Germany in 1930 and actually received a Swiss
patent (152302) Jan. 31, 1932.
Starting with alarm clock movements, the dotted plates became more and more typical for Junghans after 1930,
depending on the movement.
The machines used for the process spared out drilled holes and bushings automatically, so that these would not
have been narrowed down or closed by the planing.
Source: "Uhrzeiten - Innovationen in Technik und Design" 2011 by Stadtmuseum Schramberg, Lixfeld & Raststätter, ff.
The German patent is DRP 529786, published July 1931."