We're working out what we think - we'll add our view here!
Gossamer Gear says:
A light and fast belt for your Gorilla, an upgrade to your Drifter, or a replacement for your Murmur; this belt is as minimal as it gets.
The Fast belt can attach to the Mariposa, Silverback, Gorilla, Kumo, Camo Kumo, Murmur, Drifter, and Lonestar.
It is NOT currently compatible with the Ranger, Minimalist, Vagabond, Quiksak, and Riksak.