Hi Maurice,
to my opinion the only or most reliable studymaterial are old pictures with preferrably people wearing items and from books which are about specific expeditions, so one knows the placenames are correct. Cataloges and museumbooks are often not correct.
Besides that one cannot rely on just one picture with a name but at least three. Books as from Nieuwenhuisen, Hose, Mjoberg, Dragon n Hornbill were quite informative to my opinion.
Trading and raiding did take place and especially the iron blades where traded as also jungleproducts as hornbillivory and deerantler. Though I dont think personally that mandau taken from the slain enemy were just worn without any modification (by its new owner). I reckon the mandau is among the most important symbols worn by a tribesmember that he belongs to a particular tribe (or clan).
Even from the appearance I can often see from what area this particular person comes from.
Example; the two pictures posted by Vandoo are from a Kelabit-tribesmen <the one with the child> and the other from South Kalimantan somewhere close to Ngaju <as the mandauhandle is Ngaju-like>.
I would like to say; test me. . . .! Though ofcourse there are always a few exceptions to the rule.
Best,