The SIO Pass Through Breakout was designed to aid in development of the FujiNet hardware. All 13 Atari SIO pins are connected from the plug to the receptacle with a 2.54mm male header on the top so you can tap into the bus signals. Hookup your logic analyzer or oscilloscope to the header for debugging your SIO project.
This is a newer version that uses the same style SIO connectors that FujiNet uses. The 3D printed case has a cutout for the header pins and a label of the pinout.
These are made to order which could delay shipping a day or two while the parts are printed and assembled.