Steel strings with silk were the standard decades ago, until the hexagonal core was introduced. The sound of steel strings with a layer of silk between the soul and the wound is softer, less strident. Unfortunately, they stopped being manufactured because they were more expensive. Another problem with cheap brands was that the silk was not uniformly distributed, causing deflections.
Rivero has found an American manufacturer capable of making them with the guarantee that silk does not bunch and does not have knots in the rope.
This item includes the 2-string set.