Pak Detlef,
Can not help to confirm but there are some nice info about whale bone here: http://www.flyingturtle.org/askdrg/askdrgalapagos3.html
and interesting quote from the page below:
While whale bones are real bones, they are apparantly a little "spongier" and less stiff than most land animal bones.
One of our sources says whale bones are full of oil and very smelly while they are "drying".
For that sheath size, I think cow/buffalo bone is also possible (?), but I'm not sure either, but sure it is more readily available. I remember to hold 'tourist' keris sheath made of bone, not sure if it is made of whale bone or other material but it is bone for sure (I tend to believe cow/buffalo), similar to yours but undyed so it is lighter in color. It is heavy and smells like bone
But I don't know if we can find large enough material from cow/buffalo for the sampir.