Nuyt,
So you have no estimate of how far upriver from the mouth the Commewijne River merges with the Suriname River? I suspect it isn’t that far.
The aerial shot you provide includes the comment that the 4-inch guns of Batterij Purmerend may still be present in the heavy overgrowth near the river, but in the website you provide
http://www.dutchfleet.net/viewtopic.php?f=53&t=6966
are two color photos clearly showing an ex-USN Mark IX 4-inch/50 gun on a pedestal mount in a parklike setting, with automobiles of modern design and buildings of modern architecture in the background. Where is this gun located? In Paramaribo? Was it removed from Batterij Purmerend? Perhaps all three guns were taken out before the area was reclaimed by nature. Do you have any information on this monument gun’s location and wartime origin?
The putative 7-inch gun in yet another site appears to me to be identical to the Mark VI or VIII 6-inch guns at Fort Nieuw Amsterdam. See
http://www.dutchfleet.net/viewtopic.php?f=53&t=7033
Thanks,
Nelson