some of the machine with this finish has blackened parts instead of the nickel finish. Lots of different theories way Singer did this, most likely due to unavailability of product during the war.
here is a set of "black side" attachments, to give you an idea of what I'm talking about. These did not come as a special set, I've collected them out of other sets and put them all together. It's common to find one or two in a regular set, as they were nothing special at the time they were sold, just an alternative finish Singer used.
one other interesting bit about the wrinkle finish machines - Singer did not produce a flywheel guard with that finish - so these machines have shiny flywheel guards, and they came from the factory that way.