This is a 26-string zither from Taiwanese guzheng maker Cai Yuanhong, created under the Songbo brand. Intended for those who want a greater range on their instrument, this one yields an additional pentatonic octave or can be tuned to just shy of four full diatonic octaves.
Most guzheng music is composed for instruments with less strings. Most pieces don't require simultaneous access to all parts of even a 21-stringed instrument's range. Improvisers, songwriters, and those looking to push the instrument beyond its historic bounds may find a lot of worth in an instrument like this one, but until such compositions become mainstream I wouldn't expect the string count on the average guzheng to increase.
Source: ChineseZither.net storefront. Used with permission.