This is a black nylon string for use with string linkage rotary valve mechanisms. We sell it by the imperial yard, but can cut longer lengths as desired. One yard (91cm) could easily restring six rotors.
There are numerous videos on the internet showing how to restring a rotary valve. We don't have a favorite yet. If you do, please send a note and we'll post it.
When restringing rotary valves, I like to keep a lighter handy. If the end of the string frays and refuses to go through a key lever hole, heat the end of the string briefly with the lighter flame to prevent further fraying.