Hello Henri,
I think that there are two possibilities for this. For the first one read this thread:
http://www.vikingsword.com/vb/showthread.php?t=10454. But I doubt that this is by your hilt the case. When I look to your pictures I think to see the traces of mouse or rat bites. This I have seen many times by indonesian wooden items. Indonesians eat with their hands/fingers and when they have touched after meal for example this keris hilt the taste and smell is still present at the handle and a hungry mouse/rat will try to eat it. I think this is the case by your keris hilt. Have a look here:
http://www.vikingsword.com/vb/showth...ighlight=mouse
Regards,
Detlef