Alan, I don't know what it is, yet the examples I have seen with this feature are either Sumatran Straits dressed or are Malayan Sundangs:
http://www.vikingsword.com/vb/showthread.php?t=14292
I thought about this feature, it could be a little bit easyer way to give sorsoran a shape without cutting out the sogokan. The other influence (via Sundang) could perhaps be the exagerated Janur/even more exagerated Bawang Sebungkul sometimes seen on Moro krisses. Actually even Jeans keris is slightly going in this direction. On Henry's keris the Blumbangan has almost disappeared, yet I still see the hint.