Initially, I just purchased online, but ordered the wrong part. I called and talked to a real person, who helped me order the correct part and offered a reasonably priced return shipping label. That was a welcome change from other recent online purchase interactions.
One new 3/4" female NPT x1" male NPT in tank air compressor check valve. This style of check valve is usually screwed directly into the air tank, then the main line coming off the compressor pump is connected to it. This valve prevents back pressure from getting back to the air compressor pump once the compressor is shut off. This valve has a 1/8" female NPT port in the side which usually goes to an unloader valve on your pressure switch or compressor pump. If your application doesn't require an unloader line, this port can simply be plugged.
Specifications:
Valve Inlet Threads: 3/4" Female NPT (measure about 1" inside diameter)
Outlet Threads / Tank Threads: 1" Male NPT (measure about 1-1/4" outside diameter)
Side Port: 1/8" Female NPT (measures about 10/32" inside diameter)
Total Length: 3-5/8"
Wrench Size Needed For Installation: 1-1/2"