Fernando, We know that the Sinhala texts use the term Kasthana as early as mid 1500's; yet as you say no reference to Kasthana in Portuguese texts. may be this is evidence that indicate that the origin of the name Kasthana did not have a Portuguese root and Portuguese used a different term to describe it.
The use of Calachurro hilts as an example of the excellence of casting skill would not only indicate that the Calachurro hilts were metal but was in some way a challenging task to cast. - Like a Kasthana hilt ???
Hope one day would get to see the excellent material available to you- which sadly is rare down here.
Ibrahiim, the publication date for the source of the image is given as 1605 which may preclude later revisions. but a point to be noted is that the original image does not seem to have the side rings indicated in the later copies; and may resemble the more ancient type of Sinhala swords (similar to the ones shown in the Ivory casket.)