i would have guessed a null modem adaptor, cuz - why the hell not! but… assuming the USB to serial is a typical DTE (M) then the 9pin to 25pin would likely be (F) to (M), but the potato markings on the mystery adaptor seem to suggest M/M… so… 🤷♂️
your breakout box suggestion is a reasonable one.















as OP indicates, its a rough read - but worth your time and sharing. thanks OP.