Mostly it just needs to emphasize the left-right alignment, noting that the connector has more holes than there are pins on the mainboard.

I fussed hard with the front-to-back alignment (the usual error with most things), and didn't notice the lower pin count on the mainboard, since they were partially obscured by the HDD cable.

Oh well, no harm done, and I know about it now. Silly me.

Cheers