I don't know for sure what your '39 would have used, but on my 1951 truck the widow channel was the same all the way up the back side of the window and over the top to meet up with the division bar for the wing window. The stuff is corrugated, so can be formed to match the curve at the upper corner. It was delivered in a roll, and had to be straightened out for the rear part. Held in place by small sheet metal screws. I'll bet yours should be similar.
