The piece in the middle is the "hybrid top cap"
Here's a more regular mech mod, showing the top cap; there's a 510 interface to the topper, and a center pin contacts the battery; you can see the center pin in the pic
Now the Poldiac has a hybrid topcap, but the topper does not go directly onto the battery where it can short out;
instead the topper contacts a center pin of the topswitch module of the poldiac.
This is a mech I would recommend for a first mech.
The hybrid topcap is the second item from the right.