I don't know anything about the black cranks, sorry. They are said to be wartime production; I don't know.
That is correct; what few bikes and cars were built during wartime had no chrome; it being needed for the war effort. Don't know when the wartime restrictions ended; probably in 1945. Fits well the 1938 - 1948 timeframe for the trigger shifter.
Interestingly, this 1942 Mercury has blackout hubs, but retains the chrome chainwheel, crank, and handlebars. Best guess is that they enough of these lying around after the restriction to continue installing them.
