Bonjour Olivier,
Absolutely interesting finding.
I have in my collection the following items that are, in some way, related to your wood magazine.
In the pictures you can see a Holzattrappe, a wooden replica of the P08. In addition, a few months ago, I found out this interesting photo that shows a German soldier with a Holzattrappe during a training activity. This is the only photo I know where a Holzattrappe is shown.
Gortz states also that this item was used to fill the holster when not used for a while. This could be another use although the photo shows the use of this item for training.
Happy and safe 2006.
Ciao
Mauro