Slentho or slento, it belongs to the saron instrument family, and would be the lowest pitched member (an octave under the saron demung). Like sarons it has typically seven keys over a resonator and is hit with a wooden mallet (tabuh). However, its keys are bossed like the bonang is. It has largely been replaced by the slenthem in gamelan but it is still in use in older sets, where it is preferred over slenthem in gendhing bonang.

https://en.wikipedia.org/wiki/Slentho
https://www.wikidata.org/wiki/Q8015157
http://enacademic.com/dic.nsf/enwiki/2273651
http://web.grinnell.edu/courses/mus/gamelans/images/madu_sl2.html