Thanks Tom for your input!
I got an email from an ex-member of the forum: Dajak.
He pointed to me the following link:
http://www.vikingsword.com/vb/showth...ghlight=mandau
(thanks Ben)
This one also has this double winding.
So it looks to me that it probably is a kind of decoration, of a certain area (Kutai, east Borneo)!