I am not aware of any difference for the 1946 and the later 1947-49 versions. I checked Olsen's Library, and the part number for both the 1946 version and the later 1947-1949 motors is the same number. Part number is 671-186 Collector Insulator. Jeff Kane (Toy Train Tender) only lists the one number (reproduction part).
Whether it is a 2020 or 671, (which are the same loco but different cab numbers), a majority of the part numbers will be 671-?. Very few part numbers that would start with 2020-?.