It's about 20 years back when I last saw a proper fretless 7-string. That was in Cardiff, Wales. We weren't certain how it should be strung but we assumed nylon guitar strings would probably do an approximate job. It was on a stall belonging to a friend who used to buy and sell instruments. It didn't stay there long enough for us to play about properly - somebody bought it.
These days, I would find it an interesting instrument to experiment with! Oh Well!!!
Quack!
Geoff the Duck.