The answer is that it's either quite old or quite recent, probably the first alternative in your case.
You should try to find the article by Tromp on Mandaus from Koetei where the old hilts like yours are described. Several of the old ones have very minimalistic hilts like yours.
In later days you also see undecorated hilts but then it's probably because of that the Dayaks has converted to Christianity and the local priest doesn't want them to have hilts with design in the style of the old beliefs.
From your pictures it's hard to see the patina on the hilt.
Also it would be interesting to see the blade and sheath?
Ben has a very good Jimpul with a minimalistic hilt like yours.
And I have this Mandau (originally Ben's) that was collected by Nieuwenhuis in the late 19th C.