The pattern on the aluminum of the plug is also helpful in cases of predictable slot orientation (like a laptop, except a Dell Latitude from back in the day that had upright connectors on the back unless I'm misremembering). The side without the seam down the middle and instead a tiny rectangular cutout center but slightly lower than the other two cutouts, should face up. In the dark, the seam can be felt by lightly scraping with a fingernail.