Hi,
very interesting research. I am the owner of Spectra 1738 but have no USB307. If I will use something like circuit on the second picture could I connect it directly to Rx, Tx and GND pins for communication with? Is the purpose of AUX pin power for the connected device? Which connector did you mean, directly on the 1738 board which is for service keyboard? Does it use 5V logic not 12V? And let me say serial settings.
Thanks.