Spunjer, your kris share some similar features with this one, which perhaps even could be older, becouse it appears to have posessed a straight gonjo once:
http://www.vikingsword.com/vb/showthread.php?t=15256
The tang of this one is disturbed, yet it also seems to be between square and rounded.
We will never know, how looks like the tang of this one, also straight gonjo:
http://www.vikingsword.com/vb/showthread.php?t=13785
It seems to me also, the two linked kris are older becouse of features of Gandhik.They are close to very close to Javanese keris; the gandhik of your kris is already almost like this strange mixed Maguindanao/Maranao gandhik, which pops up so often in the last time.
Perhaps is your kris a very early Mindanao kris?