That's quite a chunk of bone and looks large/thick enough for a few crosspieces if indeed those parts are suitable.
I've been trying to match your descriptions with whatever I could observe on my crosspiece and perhaps it's of a well selected denser part of bone which also goes rather well in weight, balance and feel with the rest of sheath and keris overall. However, the minimal porousness did make me wonder if the bone is whale's afterall but there again the crosspiece is quite a chunk with other characteristics quite matching although it doesn't look as fragile.
Thanks Rick for the first hand information.