I love working with wood-took 3 years of carpentry education when I was teenager and spent many summers in a carpenter workshop while studying in University.
I adore the scent of freshly grinded wood (my teacher used to say-this scent makes you dream forrests while you sleep).
On the other hand- it is your Bolddivision 5 t. Einheitsanhaenger that reminded me wooden cargo bed is best represented by wood.
I suspect the load bed of the truck I model was produced not in Adam Opel's factory, but in Bulgaria.
Further-I suspect, that-most probably- all Bulgarian A types were purchased via the Bulgarian distributor of Adam Opel AG.
Not ordering directly, but going through local middleman was a well known and frequently used way to circumvent German unwilingness to supply its then ally with vehicles, that were in high demand by the Wehrmacht.
Bulgarian Opel distributor since 1939 was ENA- a company, situated in Russe and owned by the brothers Gabrovski.
Besides importing vehicles and vehicle frames, they produced all sorts of wooden load beds(the company also held a license for production of Opel Blitz W39 Kraftomnibuss bodies, and produced close to 80 KOM's on imported frames).
Bulgarian made load beds were reasonably cheaper than German ones.
So it was a win-win deal for both the Army and the distributor:
-the Army received vehicles, that would have been otherwise impossible to receive via official channels(and that happened many times with various vehicle types),while
-the distributor ordered only the frames and had the added value of producing the load beds for these frames.
There is a story in everything and something to learn everywhere.
That's why I love this hobby,