What Alan has noted here presents some most interesting perspective on the Sri Lanka 'piha kaetta'. While we know the seemingly close cousin of these knives, the 'pichangetti' , has been established as a utility knife, and en suite are usually picks etc......it seems presumed the piha was intended for fighting. The term for the knife is supposed to be 'pihaya' , which I believe applies to a fighting knife, but I cannot recall the rest.
Perhaps someone here might bring out thier trusty 'Deraniyagala' ("Sinhalese Weapons and Armour", P.E.P. Deraniyagala, Journal of the Ceylon Branch of the Royal Asiatic Society", Part III , 1942). I do not have mine at hand
and hope someone could check the text on that term. Also, it seems that this article (virtually the only specific reference focused on Sri Lankan weapons I am aware of) might detail the intended use of the piha/pihaya.
It would be most interesting to learn more on these seldom discussed weapons.