I believe Sea to Summit when they say:
We believe these are the lightest dry sacks ever invented. Not only is the weight revolutionary, but the hypalon roll top and siliconised finish eliminates wicking to ensure a dry seal.
Designed as internal bags rather than "strap to the rack" type. The silicone finish makes them "slippery", making them slide in and out of your bags and work together.
PU coated for ultimate waterproofing.
3 sizes; 13, 20 and 35 litres.
Colours vary, usually there is a choice between Green, Blue and Orange.
Email with first and second preference if you wish.