The connector end plates are needed if you want to be able to field replace a sled with a different altimeter or easily upgrade your electronics at any time. There are two versions of connector end plates. The first is the most common and provides just a DE9 socket connector. The second version adds four wiring posts for additional wiring points in your design.
The assembly guide here details the connector end plate with a photo of a completed wiring connector end plate at the end (the use of a wiring section is shown in more detail in the Switch Kits Assembly under Radial Switch Assembly).
Note: the assembly instructions here are strictly informational and do not take into account any customizations you may have made. I recommend having the connector fully wired when installed here and cut wires to length, as needed, for the other electronic bay parts that you connect to, such as an axial switch and deployment posts. This does not mean you can't add wires to the DE9 once installed, but this might be the more difficult way to do it.
What you need for this
Step 1 - Layout parts required for assembly
Step 2 - Put 3/8" 4-40 machine screws through connector and end plate. Secure with 4-40 K-Lock nuts.
Step 3 - Dress wires for later electronic bay installation
Completed Wiring Connector End Plate Assembly