The true determination is the types of the parts that changed that you find in your 1949 truck (windlace, emblems, ceiling liner, etc). The exact changeover date(s) probably differed by factory and based on when the old parts ran out.
If you are referring to the windlace around the door opening: the early style windlace was held to the cab with metal strips that screwed into the cab; and, the later style windlace was slid into a channel that went around the door opening. The floor-to-door cab windlace was held by a different style metal strip at the base of the door opening.
Here are two "write-ups" on differences in 1949:
information "gleaned" from the web; and,
copy of an old magazine article.
