Chris Brubeck plays a fretless Rickenbacker 4001 with some kind of black nylon strings but I have no idea what they are--I'd love to try a set if I could find them. I'm concerned to try any of the sets available though because the gauges are always HUGE! Most Ric nuts don't fit bigger than .050''-.060''-.085''-.105'' in my limited experience........
A couple examples of Brubeck's playing:
A couple examples of Brubeck's playing: