Within many Wilderness areas of the west, it's still unofficially safe to drink water directly from the stream, spring, or lake and one way to save on pack weight is to not carry water and sip as you go. These are cups that I have used in the past and continue to use on many of my trips. The "Unkown" is certainly recognizable by all and is also available in stainless steel. I lost the cap which could also be used to sip water from. I only included cups that could fit into the pocket, but I should also mention the Sierra Cup since it to can be used to sip water from the source and will hook onto a belt for easy access. They come in 1 cup and 2 cup sizes and traditionally made of stainless steel. Mine is from Japan, 2 cup capacity and made of titanium and will also fit on my stove to cook with.
On long dry stretches when water has to be rationed, the pocket cup is the perfect tool to measure out water so that your supply will last the entire dry stretch. Without pouring and measuring, it's too easy to gulp water and drink it all early on.