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Well, it seems obvious enough.
C, D, Gnd are your UART. Whichever C or D measures 1.8V is TX, the other is RX.
A, B are your EDL test points.
Normally (and only if I am right) could you short them together.
It's better to play safe.
Measure the voltage, one is 1.8V the other is 0V.
Put your DVM in diode test, black to a "sardine can", red to the 0V A or B, long power button press with USB connected.
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