A search through various Chevy Master Parts Books can hopefully provide a bit of helpful info. The exploded cab and door diagram covering the 1939-46 Conventional cabs shows the two window channel sections and nothing above that for added channel from what I can see. Further diving into the books on the parts themselves shows what looks like originally as a Parts Group 16.223 in the 1946 book: "felt" headerstrip, door window GM# 598757 which is listed in some books as 5/8" wide x 27" long x 1/4" high.
Kinda sounds like there isn't an actual channel along the top section of the window from what I can tell, just the felt (now maybe repop foam) strip?
