We're working out what we think - we'll add our view here!
Snow Peak says:
Made entirely of durable silicone, the 300ml Double Mug Silicone Lid is a cover designed to transform the Ti-Double 300 Mug and the Stainless Steel Mug 300 into travel-safe beverage containers. The snug fit secures easily onto the top of the mug with an open lip for drinking. To clean, simply remove and wash with any dish detergent before air drying.