Anthony, I think you will find that the wood is burl teak (jati gembol), it is a wood that has been used for a long time for wrongkos & also for hilts. When they were clearing the forests in Borneo there was a lot of it around, but in recent years it has become very hard to get. About ten or twelve years ago I had an order in for over two years to get two pieces of jati gembol big enough for ladrangan atasans.
Here is an example of a different type of burl, this is thuya burl from Morocco.
When this wrongko was made, back around 1990, thuya burl was very difficult to get hold of and quite expensive. I just did a quick search and found that it now is a bit easier to obtain, but still not cheap:-