Could Use Some Help with these Brass Ducks

I got this little set of brass ducks at a recent auction because I thought they were so cute. They are not marked. Were they made in India?

The mama duck and the smallest duckling are solid. The other ducklings have a place to screw something into the bottom. Do you know what this is for?

Should I polish these? with Brasso?

 

So sweet.
Maybe mama duck and the smallest duckling are one set, and the four others another? Mama duck and the smallest duckling seem to have more pronounced eyes, the other ones just have dimples for eyes.
Or the four are actually two pairs?
The tray is not part of the set imo, or did you just use it for display.