Bruce,
The model was done by Marcos Serra (panzerserra), so you could try to contact him to ask if he based it of a picture.
http://panzerserra.blogspot.com/search?q=huw
However....I am betting it's a bit of a a guesstimate and a mix of decals from other kits, as not only the "CZ" is misisng, also the number is not very kikely for a CMP.
That being said; I wouldn't be suprised if a period picture does turn up with a star on the sliding hatch.....as you say, it's a rather nice large flat area.....and as the attached picture shows some 3Ton trucks even had HUGE starts painted on top of the canvas for D-day....not just a smaller one on the cab roof.
source:
https://www.historynet.com/unloved-u...nsable-lst.htm
I though I had photo evidence here....but, it's most likely a HUA with a Red Cross in stead, just like the one in the IWM picture below.
source:
https://www.iwm.org.uk/collections/i...ject/205155925