Peter,
I have also looked into this matter.
All my Dutch topo-maps only show the straight borderline towards Emden and
don't give the border in the Eems.
I have a German map though (Die General Karte, scale 1:200.000) which,
respecting the straight line towards Emden, leaves almost no space between
the borderline and the Groningen mainland.
Inspired as I was, I have uploaded a new page of my humble website, showing
the borderstones near the Dollard.
http://leden.tref.nl/~veenhh/borderland/nederland/dl-pa-gn.htm
gl
Harry ten Veen