Just a note to let you know I added the dimensions of the Mystery sword and the other old Moro kris to the pictures above. As Bill commented, it is a very consistent feaure to find the blade lengths on these early Moro kris to be 18 +/- 1 inch. These two lie within that range, with the blade of the mystery sword being 18 inches from the tip to the hilt.
The mystery sword is a little thinner and slimmer than the other kris. Not sure this adds anything to the discussion. We know that it is corroded and may have lost some thickness relative to the better preserved sword. It is also possible that kris became generally "beefier" over time. Certainly the late 19th C. Mindanao kris became much heavier than earlier examples -- perhaps the mystery sword predates the other one shown beside it.
I think Bill's comments come closest to my own thoughts about this mystery kris.
BTW, I keep this one out of my bedroom and certainly not under my pillow. Who knows what stories it has to tell.
Ian.