From their website:
Assembly of 3 strands of moderate shine.
Silkworm Silk 1003™ is an assembly of 3 strands of moderate brilliance, made from schappe silk (combed silk), it is perfect for all uses in the field of needlework, available in an inimitable range of 290 colors.
In 1890, the Silk Card 1003 had 2688 different colors divided into 107 ranges. It is this card that serves as a reference to add new colors. Our current color chart comes from it.