Well – I’ll answer my own question so the next poor soul doesn’t have to go through the pain. Took a multimeter and decoded the pinouts which have zero resemblance to the published pins.
MX88 DB15 / HD15 Adapter cable pinouts.
Pin 5 Ground (DB9) —> Pin 11 and Pin 9 (DB15/HD15) — only pin 9 is required.
Pin 3 TX (DB9) —> Pin 12 (DB15/HD15)
Pin 2 RX (DB9) —> Pin 13 (DB15/HD15)
It’s now working!