It looks like you're asking about a 1/4" x 3/8" compression adapter fitting. Here are some details about this type of adapter:

• Material: Typically made of brass, which offers durability and corrosion resistance
• Connections: One end connects to 1/4" tubing via compression, and the other end has a 3/8" male BSP (British Standard Pipe) thread
• Applications: Commonly used for connecting copper, brass, aluminum, or plastic tubing in various systems, including water, gasoline, oil, vacuum, and air lines
• Pressure and Temperature Ratings: These fittings usually have a pressure rating up to 300 psi and can withstand temperatures ranging from -65°F to 250°F