Adria bathroom faucet aerator installation

[Tim McGann]

My bathroom sink aerator fell out and I am hoping someone here can tell me how to reinstall it. It isn't the normal type that simply screws in, it has no threads. There is a groove around the top and a notch that lines up with a piece inside of the opening, but I cannot figure out what keeps it intact. There is a small "window" in the side of the aerator with adjustable numbers but I don't know what that does nor how to adjust it. Perhaps something that holds it in place went down the drain?
Sorry the attached pictures are not too clear. Thanks for any help.

 

[Landy Andy]

Is it retained by the O ring?

 

[Tim McGann]

I tried pushing it in and it was very hard to do. As soon as I turned on the water it fell out again. Perhaps I didn't get it in far enough, maybe there is a tool for that. It's hard because the sink is in the way preventing a better view.

 

[mitzimad]

put you phone on video sefie and view the tap from below

 

[Landy Andy]

How clear is the aerator?

Are they push fit so when they get blocked they pop out?

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[Tim McGann]

Yes it had some foreign matter on it and that's exactly what happened, it popped out. I think you have identified the cause and the style for me and I'm grateful. I just don't know how to put it back in.

 

[IRHP]

They just pop in . And out sometimes

 

[Spriddler]

They just pop in . And out sometimes

Just like Ronnie Biggs.

 

[Tim McGann]

Okay, I'll give it another go. Thanks