Thank you, Manolo, for your suggestion on the 'designing' aspect. There is, actually, a 'pre-formed' greneng called 'greneng wurung'. Wurung or durung means 'belum' in Bahasa and literally means 'not yet', 'unfinished'. In this case, it could be translated as 'unperfect greneng'. But this kind of greneng has it's own problems. It is very possible that this forms were caused by the worn-out blade. This form also, very likely, to be found on the lower quality blades. Thus, incapability of the maker should be taken account. But it is quite tempting to think that the greneng wurung is the predecessor of the full-shaped greneng.
David, as you suggest that we should find the cause in Hindu cultures, why don't you consider the kukri's cho ? This notch in the lower part of kukri has similar form with greneng/ ron dha, and it also has symbolic meaning(s) and 'lack of functionality' as a tool/weapon.
http://www.himalayan-imports.com/khukuri-history.html
Please note while I'm suggesting some relations between cho and greneng, I keep my opinion neutral regarding this relationship
