Black 2x4 curved top piece of assembly This piece does not have an identification number on it. It appears to be part of a larger assembly. At each side are two 'legs', neither of which have stud receptacles, so it doesn't hook on to studs. 

I'm guessing it slides down inside something as a top piece.

Can someone tell me what element it is?
Technically you don't ask what that element is, but I'm guessing that's what you want to know.

I think it is:
LegoFabuland Container, Side Bags / Saddlebags