psu cable missing pins inside socket

Solution
That's normal.

For the ATX cable:
240px-ATX_PS_signals.svg.png

The Pin #20 is formerly the −5 V, now absent in modern power supplies; it was optional in ATX and ATX12V v1.2 and deleted since ATX12V v1.3.

https://en.wikipedia.org/wiki/Power_supply_unit_(computer)
https://en.wikipedia.org/wiki/ATX

For the PCIE cable:
PCIe_pinout.png

The Pin #2 is optional (but may be supplied with +12V on some PSUs)

MySlicLife

Prominent
Apr 19, 2017
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0
510
Some pins may be missing because of the way they attach to other components in your system. But I'm not sure with that specific power supply. I would suggest finding out where they belong plugged in and see if there is the male end of that pin there, if there is then it was a manufacturing error on Seasonic's end and I would contact them or try to get a replacement, if the male end is missing then it is supposed to be like that! Hope I helped!