Traditional guitarrón mexicano
This is an unusual instrument to be sure, but not really that impressive until you actually see someone holding one.
I came across them in Southern California where there has always been a strong Mexican sub-culture, where mariachi bands wander the streets at night playing folk songs outside restaurants and bars.
The guitarrón is a massive six-stringed fretless acoustic bass guitar, with three steel and three nylon strings (steel for the lower notes). The traditional wooden peg head is a struggle to tune properly; modern versions use mechanical machine heads.
It’s a monster to hold and a bastard to play. Although it’s quite loud, you need a grip of iron and a strong plucking arm to compete with a couple of trumpets and six or eight violins.
The odd shape of the back is to facilitate the playing angle of a standing musician, but I have to say it looks like this would be a real pain in the neck after an hour or two.