Is the reverse postorder of a digraph's reverse the same as the postorder of the digraph?
I wrote a short example to test the hypothesis and found out that the reverse postorder of a reverse graph is indeed not the same as the postorder of the original graph. 
Consider the following graph:



Link to source (archive of image used in original post)

Postorder for the above graph is: 0 1 3 2 4 5.

Postorder for the reverse graph is: 4 5 0 2 3 1.

Reverse Postorder for the reverse graph is: 1 3 2 0 5 4. 

You are thinking on the correct lines, but what we should understand is that the postorder of a graph is not necessarily the reverse of the postorder of the reverse graph. This is because of the way we iterate over the vertices when calling the depth-first search. There could be different solutions to the same problem, given the iteration method is different.

And because of this, the reverse postorder of a digraph's reverse is not necessarily the same as the postorder of the digraph.