Here is a detail pic of a cab 13 water truck from the NIMH archive.....sadly a bit hard to see, but this one still seems to have rubber flaps fitted, while the servicepub CD suggests that some late war water tankers might have been fitted with canvas flaps.
source:
https://nimh-beeldbank.defensie.nl/b...eld=waterwagen
(other two pics in previous post are of a C15A cab12, by the way!)
Alex