From the factory these are plated and as you probably know the ends are over molded in soft black rubber where they touch the plug wires.
I have always seen the holders with the hollow side up. Also be aware there are 3 different plug wire clips.
1. Standard 3 prong, very common, two per engine.
2. horizontal 3 prong and one vertical, making an “L”. One per engine.
3. the last one has another feature that holds the coolant temp sensor wire, if your car came equipped this way. Also one per engine.