For those that don't know, these are handmade strings. They are not mass produced. The core is round wire instead of the six sided core that most strings use. This allows a lighter gauge with the same response and because there are no gaps under the windings, as there are when the core is not round, they have a great sound that lasts. They will custom make strings for any instrument with any desired string end. They are pricey, but due to their performance, many of the top players are using them.
Once upon a time, all strings had round cores, but because of the inability to have good quality control on the winding, they switched to the six sided cores.