I've attempted to figure it out myself using a USB Serial TTL board and header cable with no luck.
You have only searched using that cable? That can be done better.
First switch off the box, and use a multimeter to measure the resistance between all pins and the ground. This will give you a number of pins with a zero resistance, ground pins, a number of pins 50~500 ohm, signal and Vcc, and maybe some pins with an infinite resistance, these are probably not connected.
Then switch the box on, and measure the voltages of all potential signal pins. All pins with a voltage >5V should be ignored (and take care against shortage). A 3.3V pin can be Vcc, or can be signal. A voltage between 0 and 3.3V can be signal.
Ultimately you have an oscilloscope handy, which you use to measure the potential signal pins while powering up. TX should spit a boot log, which can be recognized. But an oscilloscope is an expensive device, so I can imagine you don't have it. In that case you can use a cheap piezo buzzer. They can be found in cheap alarm clocks, watches, ...
Connect one wire to the ground, and the other to a potential TX. If it starts beeping while booting up, you've found TX.
RX is more complicated, as it can only be detected by offering a serial signal, but is in most cases next to TX.
BTW, of course it's recommended to do this searching without any disk inserted. The disk doesn't like to be powered up and down much.