I found it at a brocante for 5€ dont know what it is for though, it is good quality stainless steel

Bottle opener?

Stainless? Citroën?? 

It looks like an ashtray to me.

Does it come apart? It looks like that back piece is in an angle.

How big or litle is it?

It looks like a botle opener. Strange, french people are normaly drinking wine

It doesnt come apart no and it is 70mm dia and 25mm high
A bit over kill for a bottle opener wen but I had thought of that and in fact tried it out, it sort of works

Screwbox for the mecanic who has to take apart the car (again )?

I suggest you go back to the person who sold this and start your investigation there. Where did this person got it?
I think, somebody was bored and made something useless out of a hub cap. Only because that back part just fitted.

Here in France private sellers are not allowed to do more than 2 brocantes a year so dont tend to be traders. Hence, when I asked the trader, she just shrugged her shoulders and said it was in the garage and her husband didn’t like Citroen cars, at that shocking comment I slapped her….. NO NO! slapped a 5€ note in her hand and trotted off clutching said item