From the Knit Rowan website:

Knit this ladies’ classic sweater with low scooped back from Still. Designed by Kim Hargreaves using Kidsilk Haze (mohair and silk) and Fine Lace (baby suri alpaca and fine merino wool) worked together, this knitting pattern is suitable for the knitter with a little experience.

Two strands of laceweight held together to achieve fingering weight gauge.