It’s called a head fairing. The purpose is to smooth out the air buffeting around the occupants heads. Here’s a picture of a 1935 Deusenberg with them.

Looks like a tonneau cover. It is added behind the seats and covers the gap between the seats and the trunk / rear fenders. It acts as a hard protective cover for the convertible top, instead of a vinyl or fabric cover that snaps into place like on many convertibles.

its a fiberglass cowl for the roll bar…..