Sangkuh, IMHO, could be an alteration and looks very identic to word 'Sangkur', a Javanese and Bahasa word, which in English means, as we expect, bayonet. I haven't heard here in Jogjakarta that this kind of blade would be classified as 'sangkuh', but I would except it, as I cannot give any other alternate names, while this kind of dagger quite familiar
I had also found 'true bayonet', with a ring for mounting the blade onto the muzzle, (or 'a round elbowformed tube to the socket' as Henk describe) and with a splitted pamor, exactly like Henk and Alan have. Could it be a Javanese made bayonet ? But for sure, it is not a tombak/spear. Tombak, as Alan said, need a 'methuk' to be mounted onto the spear shaft, in Javanese style spear. In fact, looking for a good methuk is the first step on assesing the tombak blades. the 'lingiran' tombak usually has an equilateral triangle shape, with a long pesi, and a methuk. The more common is the quadrangular, phillips screwdriver's shape which may leave a nasty ' + ' shape if you let it come into your stomach
This quadrangular blade usually called 'cipiran', shaped like cipir, a kind of tasty vegetables.
But if you ask me to vote, I may vote for modified bayonet. A flat side with a fuller and marking on one side only is a typical to this blade, as Henk pictures show.