Thanks! All cables say Type 3. I do have another MB cable but it pins differently, so it doesn't fit. I tried the paper clip trick and when connecting I hear a "click", followed by the PSU fan spinning for a second or two, then it stops. When I unplug the clip, I hear the "click" again, followed by the PSU fan spinning for a second or two, then it stops.
EDIT - Instead of "click" I would describe it as "tick" and then "tock".
EDIT2 - I have a multimeter, so if you want me to check continuity to rule out the ATX cable, I can do that... LMK