[VVV]

Thanks for publishing the picture of the sea cow skeleton!
It's easy to imagine that if you see such a skeleton on the beach you might think of a sea dragon, if that is a part of your world-view's probabilities.
In Hornbill and Dragon, by Bernard Sellato (1989), his, maybe a bit simplified, cosmology of the Dayak is that the Upperworld is the Hornbill and the Underworld the Dragon. On page 44 he writes "In the arts, other reptilian or aquatic animals - crocodiles, lizards, turtles, snakes, frogs, and fish - are equal to the dragon." On the same page he also claims "The Bornean dragon corresponds to the Indian makara water-monster..."
So why don't we see dragons on the parang hilts? Actually now and then you see a lizard hilt, even if it's quite rare (enclosed are 2 examples from Hornbill and Dragon). And below them is an example of a Makara on an Iban Jimpul from my collection. But these seem to be more of an exception than a rule.

Back to the hypothesis on if Malay beliefs somehow might have influenced Ibans when they started to travel by sea for their raids. I have used some quite old sources to investigate this because already in the late 1800's the British quenched those raids. Actually it was the same time that the Langgai Tinggang and the Nyabur were more commonly used as Iban war swords.
It is evident that the Ibans, who originally came from the inlands of Borneo, became pirating Sea Dayaks (instead of River Dayaks) first after they had contact with the coastal Malays of Sarawak. Here are some supports for this:

"The Dyaks of Sarebas and Sakarran, a brave and noble people, were taught piracy by the Malays who dwelt among them. These Dyaks were always head-hunters, and used to pull the oars in the Malay prahus for the sake of the heads of the slain, which they alone cared for. But, in course of time, the Dyaks became expert seamen. They built boats which they called bangkongs, and went out with the Malays, devastating the coast and killing Malays, Chinese, Dyaks, whoever they met with."
Sketches of Our Life at Sarawak, by Harriette McDougall (1882)

"The Sea Dyak's history likewise shows a growth of headworship parallel to the increasing importance of war. When they were mere agriculturalists. fighting only among one another over disputed ownership of land, they used to take the heads of their enemies slain in these battles; but it was not until the Malays taught them to grow rich by piracy that the passion for head-hunting became deeply rooted, and that they went on expeditions for the avowed purpose of getting heads."
The Influence of War and Agriculture upon the Religion of Kayans and Sea Dayaks of Borneo, by Margaretta Morris (1904).

Below is also enclosed a part copied from On the Wild Tribes of the North-West Coast of Borneo, by Lord Bishop of Labuan (1863) - Pasted Graphic 2.pdf.

The Lord Bishop also confirms the practice of secret languages among the Sea Dayaks - Pasted Graphic 3.pdf.

In The Relations Between Men and Animals in Sarawak, by Charles Hose; W. McDougall (1901) the authors confirm that the Iban have a lot of contact with the Malays and that this contact has influenced their belief system - Pasted Graphic 4.pdf.
They also give another reason why maybe an Iban would like to have something from a sea cow as a hilt and why it is so rare to see it - Pasted Graphic 5.pdf.

Michael