By "first" I assume you mean the 1914 Admiralty Pattern cars with the turrets.
The actual hole appears in photos to be "oval". I put "oval" in quotes though because they are not true ovals. They appear to be a vertical rectangle with a semicircular top and bottom and straight, parallel sides. Sorry I can't post a pic of the real thing at the moment, but here is a patent image of a slightly later port.
On the 1914 cars the cover appears to more or less match the shape of the port opening itslf, while on later cars the cover is distinctly asymmetrical.
