Hello Henk,
I thought a Dutch coin might fit there simply because a similar piso posted by Charles S had one
http://vikingsword.com/vb/showthread.php?t=1342
and a similar albeit none-piso sword also has an ornament on the butt of the hilt
http://vikingsword.com/vb/showthread.php?t=1907
The hilt on my piso has a hole on the end so meybe something was inserted there. I guess anything could fit there.
Emanuel