Keris with strange ( to me ) blade

[fernando]

Please help here,
This is a small keris i have just bought.
Its 8 inches blade has a strange particularity; instead of having its sides regularly plane it has, how to say, some hollows or waves . The edges are almost not afected by this efect ... the hollows are more noticeable in the middle, like some deformable material you mould by pressing your thumb upon.
I hope this is visible in the pictures, as i wouldn't know how to put it in a propper english .
Seller has dated the blade XVI century and the handle XIX century.
I would be much obliged for your coments, namely on the blade typology, attributed age plausibility, and whatever you kindly offer from your knowledge.
Fernando