Catalog entry at the Musée de la musique in Paris

This is a roughly cello-sized instrument with 6 playable strings and 12 sympathetic strings, 18 in total. The catalog labels this instrument a “Viola Bastarda” which usually means a virtuosic and aggressive playstyle on a regular Viola da Gamba, not a type of instrument.

https://en.wikipedia.org/wiki/Viola_bastarda

Sound sample of the bastarda playing style on YouTube

The soundholes in the form of flaming swords, the rosette, the sympathetic strings and the blindfolded head on the pegbox suggest that this instrument is part of the “d’Amore” family, a Bass Viola d’Amore.

https://en.wikipedia.org/wiki/Viola_d%27amore

So far I haven’t found another instrument like this, maybe it’s unique?