I can confirm. Yes, it's the part that helps center the PF. Not mission critical but I would put it back in when you have the opportunity. Their should be one on both sides of the cab.

You can do one of two things.

I personally loosen them, install the mirror blades and then re-install them. This helps keep the mirror blades in place while installing them as well as holds them tighter against the cabinet.

On tighter games (less clearance for the PF and the cab) I will install the blade over them. here is a picture from my MET. It had a tight tolerance so I had to install the blade over the lip of the centering piece.

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I prefer to have them on the outside of the mirror blades. It holds the blades in better.

In the case of my MET the PF was to close to the cab so doing it that way wouldn't allow me to lower the PF all the way. Switching it worked fine as it was a smooth transition.

Same purpose. The bottom is what aligns the PF. You can put your blades over the top of them and be perfectly fine.