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 in the middle so you can tap into the bus signals. Hookup your logic analyzer or oscilloscope to the header for debugging your SIO project.
The 3D printed case has a cutout for the header pins and a label of the pinout. The connectors are edge mounted on the PCB and use the same pins as FujiNet.
These are made to order which could delay shipping a day or two while the parts are printed and assembled.