Hello Detlef,
Congrats, that's another good grab!
Yes, definitely a Batak piso. As you mentioned, this blade belongs to a family including sewar-like pieces and what the coastal Malay of the Straits region seem to call tumbok lada; among the Batak, there is a bunch of these with either the bulky ivory hilts or figurally carved pommels.
From the carving motifs, I'd be inclined to lean towards a Karo origin; Simalungan can't be ruled out though. The motif of the silver sleeve has also been seen with blades that seem to suggest a northern influence. Thus, we need to keep Dairi/Pakpak and Alas in mind, too. There even may be a Toba link here - so, we're going pretty much full circle...
This is a topic I'm currently looking into; more data will hopefully allow to reach more reliable conclusions. It will definitely take a while to sort things out though.
Regards,
Kai