The co-ordinates are part of this protocol (art. M5.5), but were not
included in the online version for technical reasons, it says here.
They can be found in the Belgisch Staatsblad/Moniteur Belge, 26-06-
1996, p. 17571-17592.

The enclaves are not demarcated in any way. Cf. Art. M3.c:
"As a deviation from the Regulations for the establishment of
boundary markers, as added to the agreement of August 8, 1843, no
boundary markers will be established. Since the boundaries of the
enclaves will be unequivocally established in the national co-
ordinate systems of both Belgium and the Netherlands, the annexed
maps will be drawn using these systems, and the production and
placement of cast-iron or hard-stone boundary markers according to
the regulations will be extremely costly, it is decided not to place
these boundary markers. The placement of boundary markers would,
taken into account the buildings that are located on many enclave
boundaries, even be impossible in those cases."

Which gives a direct anwer to the question about the status of the
metal pipe near or at point 5109.

Peter