The green Duplo 2x4 is included for scale. I am guessing this is for a construction set, but I can't find where the grey circle goes with any of the other Duplos.

 

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It might not be lego.  I'm not familiar with any piece like that, and lego typically mounts their crane or tipper parts differently, so that they cant be separated. 

Have you looked on the bottom of either the gray or red part for a part number?  For example, on the underside of the green duplo brick you should be able to see "3011" as the part number.  Are there similar numbers on the other pieces?  If so, you should be able to search bricklink by part number to find the set it came from.

HTH.

You are correct, it is not a official Lego piece. There isn't a part number underneath, and it doesn't say Lego inside the circles (not sure what these are called).

I will have to figure out what kind of block it is. Official Duplos fit nice and tight on the top, so it would be great to be able to add this to our building sessions.

Thanks for the help!

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