To be honest I didn't find any answer to the question in any book. However IMHO there wasn't as many cockpit tops as you mantioned. IMHO there were two only ( perhaps 3 ). And always there were two windows only.
The first one seen on the pre-war Do-18s with the ring antenna behind the cockpit. It seems that the starboard window was a half smaller that the port one. The starboard smaller window could be caused just by the antenna mount there. But it might be a light trick because the second pic I found at Alamy site shows the two windows there. But in the case, the antenna seems to be moved more toward the starboard edge of the fuselage. However there were the kind of curtains for each window including the side ones to protect the crew against the sunlight. So it is very likely the appearance of the small in a half starboard window was caused just by a closed shade. Also it may be the variant A depicted below.
And the second variant seen on planes without the ring antenna. And here there are two sub-types... the easy noticed difference is the layout and length of the stiffeners on the roof.
the A sub-type similar to the pre-war planes .. the starboard glass looks like it could be either slided back under the fuselage cover or move up and side while the port one has the protuding frame for sliding of the glass ... additionally the two stiffeners on the roof are of different length.
the B sub-type with two windows looking the same including the same sliding way there ... the stiffeners are shorter and of the same length. Additionally the darker appearance of the two windows in the top shot might be a result of the blinds I would say. Please notice that the side one is also closed.
A close up shots I grabbed from a documantal film, reveal that the starboard window wasn't slided back rather but it could be moved up and side. I would say there are seen hinges at the frame edge and the thin bar of the window frame at the middle of the window rectangle. The port one had a kind of additional protruding up frame with guideway what indicates it could be slided back. And also it looks like there was the thin bar frame in the middle.
And here is the third variant ... I'm not sure if the side window seen in the top pic isn't the result of ability of moving it from the top to side. but in the bottom shot the thin bar being in the middle of the starboard window can be noticed too.
The pic source: the Internet.