Thank you, Alan. This is a very interesting piece of information.
I don't know anything about keris making and the information available on the web is rather unspecific. I wrongly assumed that the tang was part of the original package like it is done for modern european damascene techniques as shown in this video:
http://www.youtube.com/watch?v=5F27WzpIAiQ
Of course it is also possible to use a compact package and weld in a seperate tang during the final folding step of the package. This saves precious metal and is a lot easier than performing an inlay repair on an old blade.
Another benefit is that the smith can not only make up for wrongly calculated material but has good control of the pamor at the base of the blade. Drawback is that if too much material is removed the outline of the welded-in tang becomes visible.
So the rectangular shapes we see might indeed as well be the result of the original manufacturing technique.
Best Regards,
Thilo