This is a Sioux Chief Mini-Rester Water Hammer Arrester (660-TC0) used to stop banging pipes by utilizing a pressurized cushion of air to absorb water pressure spikes - it connects to 1/4" water pipes and is primarily used to prevent pressure spikes in the water lines of icemakers.
The Sioux Chief 660-TC0 Water Hammer Arrester uses a compression tee connection and dual o-ring piston to separate the air cushion from the water system to ensure permanent protection from waterlogging.
This water hammer arrester can be installed vertically, horizontally, or any angle in between. No access panels are required to use this part. It was manufactured in the USA and is also known by part number 660TC0.