Well, Joe, please take every factual statement you hear on FIF with a huge chunk of salt if you may...
Malaysia derives from the British colonial sphere of influence - the Dutch did not enter there.
The Dutch klewang was never based on any indigenous blade. During the Aceh war(s) and resistance periods, Indonesian conscripts (mainly from the Moluccas and Madura IIRC) found the performance of the issued Dutch swords wanting, especially in close quarters - no big surprise there. Them resorting to local Aceh swords and other traditional blades initiated the development of the Dutch-Indo klewang.
These are thin and slender blades - you'd need to join at least 2 klewang blades to obtain a mandau blade. Thus, klewang blades are an unlikely source.
Have you followed up on Wilkinson and other British suppliers?
Another possibility might be modern Sunda smiths moving to Borneo and setting up shop there (most likely after independence and possibly transmigrasi).
Regards,
Kai