I actually made this for the isolated variant, because I didn't want to complete the matrix by making an isolated RS-232 variant. What it is is simply an RS-232 level converter and a DB-9 on a board that plugs into the 10 pin connector for the TTL level variant.

It doesn't matter whether the power is jumpered for 5 or 3.3 volts. The converter chip works either way.

It's designed to have a 5x2 .1" DIP header mounted on the bottom side of the board, and plugged into the UART board in such a way that the DB9 is facing the opposite direction of the USB-C connector.