Pre-WW2 regulations are very hard to pin down! Although one might think the FIA was the place to start, and they do have an archive, it's simply inaccessible.
The RAC Library in London might be worth a try. And I'm told the RAC of Belgium are very helpful.
I know it may make me sound like Oliver Twist, but if they can go to the trouble of scanning the appendices you'd think they could scan the whole bloody booklet!