Dear Alan and Willem,
Thank you for your interesting comments and again I have no academic background nor proven evidence that these hilts are made from hippo ivory but wish to reply as follows:
. All the ivory hilts supposedly made from hippo ivory which I saw or own are either from Sulawesi or Sumatra/ Malaysia, so the Javanese or Balinese dealers or experts are not very qualified to identify them IMO.
. The trading between Bugis sailors and East Africa or Oman (which controlled Zanzibar and Tanzania ports until beginning of 20th century) was well established in the past, I will try to find more written evidence.
. The external dotted line is connected to the interstitial zone found in hippo ivory and not other species as far as I know.
. All the hilts supposedly made from hippo ivory which I saw are old (circa 100 years or more) and the hippo ivory trading with Indonesia vanished long time ago so I doubt that you can find any Indonesian hilt maker familiar with the materials nowadays (but probably in China).
. For those of you who like myself are familiar with such pieces, the materials looks and feels very different from the other ivory species from it high density, white colour, and very polished aspect, and it ages much better than elephant ivory for instance (no cracks). Spermwhale tooth ivory is also different because its colour is darker inside.
Well, this is my last attempt to convince you on the subject and I will welcome the opinion of experts.
Regards