Pepijn Hendriks wrote:

(...) At the tourist office in Lublin, I've found topographical maps
(1:100,000) of the area. A scan of the border at W³odawa and the
tripoint is attached (...)

It looks like the tripoint might not be a thalweg junction after all.
The dash-dot line is not everywhere on the border: the byua boundary
seems to follow a path over some length here, and the purple shade is
much more an indicator for the exact location of the border.
Unfortunately the byua purple boundary shade loses itself in the bypl
and plua shade (how poetic this all sounds). But I suspect the byua
boundary never joins the Kanal Moscickowo (if I read it correctly on
the map - diacritical marks not counted) as it approaches the river
Bug.

By the way: great pictures, taking into consideration the awful
weather conditions!

(...) border marker 1136.jpg:
A close-up picture of border marker #1136, a couple of hundred metres
south of #1137. This border marker is not indicated on the attached
map. (...)

The border marker is there alright. The number isn't.

(...) My guess is that the abbreviation _ceram._ has got something to
do with the border marker, but I've not been able to ascertain its
meaning; it's not in the list of used abbreviations. (Any ideas?)
(...)

My idea is that it might be the location of a (now disused??)
ceramics factory. The abbreviation ceram. definitely stands
for "ceramiczny". But it could stand for something else as well, of
course.

Peter S.