Randy---
The body of your gun is definitely German, not Belgian. It is typical work of the best Suhl makers, though this gun is certainly not anywhere near the level of their best work. It is a bit odd in having just that little blipp in the juncture between the action and the wood. In the cheapest boxlocks this will be perfectly straight. In the better grades it had one or two roundings machined in, and it was scallopped in the most expensive. So this is peculiar.
The side indicators are an upgrade item.
I think your interpretation of the markings regarding the dates of proof is right.
As to the maker, this seems to be a persistent mystery. German guns exhibiting traits such as found on yours are not uncommon. I have a few of them and I have see a lot. But no one seems to know who made them.
Good stuff, though.