Yes some of them are dutch.
The Z mark is definately not the 800/000 mark, it's either the Z I or the Z II mark (Z I is for 925/00 and Z II for 835/000). The Z in the 800/000 mark is centered, so this must be a I or II mark.
So my guess is that the 830s is actually a 930s
As said the DA+ mark is for Aubert, one of the major silver importers in Holland (they are still going strong).
I should note that a "+" behind a responsibility mark indicates an importer (I believe that has been changed recently, but usually it is).
A classic hallmark which I was quized about on my exam.
I'll give Aubert a call in the morning, I talked to the father of the current director a few years ago.. nice guys, so I'm sure we can find out who the maker is.