Well, MK, what you are showing us is what is known as a keris buda and is believed to be one of the oldest forms of keris. However, judging authenticity of these (and keris in general) from internet photos is always tricky, especially since so many modern reproduction of keris buda have been made and then artifically aged. That being said, the form you are showing would not be the oldest type as it has a seperate gonjo, the "guard" piece along the base of the blade.
On the down side the erosion of this blade does look to be quite even, a possible sign of artifical aging. On the up side the close-up of the pesi (tang) does seem to show some remnants of hair which was a common old way to secure the hilt tightly on the blade. Others here may be able to tell you more definitely,but without a hands on examination you may never know for sure.
