This is GSI's Bottle Cup. It's a stainless steel cup that fits the bottom of most 32oz bottles, in this case it's a standard Nalgene bottle. Now, I have to ask you, does this remind you of something else?
So when I saw the GSI cup I had to grab one. I'd actually ordered two canteen/cup combos online a couple days earlier, now I wish I'd waited. I already had two Nalgene bottles in my GHB, this cup is a perfect addition. Stainless steel, inexpensive ($8.99), well constructed, light weight, and it hardly takes up any space at all. For the space/weight conscious prepper this cup definitely deserves a look.