That is the winter safety. During the Korean war, some M1D's were changed over to use the winter safety and winter trigger (which was a lever attached through the rear takedown hole in the trigger guard) for use with heavy gloves. They were never general issue, and it is safe to say that almost all the winter safeties seen today are not GI, but were made up (or converted from standard safeties) to sell to M1 rifle collectors.
Jim