I'm more used to 13 cab, 15cwt and above. Since you working on earlier and lighter, my thoughts might not help you. The later, larger trucks all seem to have been factory fitted with two sliding metal panels, some were later replaced with sliding plexiglas or glass panels, most likely after release from service.
Some of these parts were offered for sale in the following thread (I'm quite sure they sold):
http://www.mapleleafup.net/forums/showthread.php?t=8071