Hello David and Detlef,
Sorry I did not see the latest pictures from Fheng before writing my latest post and I agree that the origin of this blade remains uncertain.
Regarding the sand-like texture of the Sumbawa blades, I am of the opinion that this is not by far an universal aspect, you can refer to "Hartadi keris Lombok" site for instance which shows several krisses found in Sumbawa which do not meet at all this specification.
It seems to me that the krisses from Sumbawa are found in various qualities and specifications (dapur, pamor, iron, etc). It is possible that some blades were imported from Sulawesi or Lombok (or made by smiths coming from these islands), and that the pieces with sand-like texture and no pamor (tangguh Kupang) were produced by local smiths with inferior materials. Furthermore it is likely that the krisses from the Mbojo and Sumawa ethnic groups are different since they had little contacts, anybody could confirm?
Regards
Jean