Okay, I have a theory.
Suppose the puzzle creator generated an aztec code with the password which is 19x19 pixels.
Take a copy of the original code and rotate it 180 degrees.
Now cut out a triangle from the lower right of the new rotated image (defined by top right, bottom right and bottom left corners). Paste this over the original image.
Now, the pixels along the line defined by top right and bottom left would have been swapped. But it looks like the pixels in this image are mirrored. So that's 9 pixels that were left untouched (if you don't count the very center pixel). That would mean exactly 181 pixels were 'lost' in this process.
19+18+17+16+...2+1 - 9 = 181
Now suppose you wanted to encode the 'missing' lower right corner in the pixels of the newly created 'invalid' aztec image. You would need 181 pixels to encode on/off, right?
Suppose you went through this process and discovered that the now invalid aztec code you created (original + pasted part) only had 179 pixels set (just because that's the way the encoding went for the top left part). What would you do? Well, just add 2 pixels to the lower right corner since that's the section that's going to be restored anyway. (That would explain why there are 2 extra pixels in the bottom mirror part.)
So, in short, I believe the goal of this first step is to restore the original aztec code. There are 181 pixels that need to be restored in the lower right corner and I believe that information is given in the 181 pixels of this image.
The only part that is troubling is that you only need on/off information for those 181 pixels so the 3x3 grid and triangles are still a mystery. This theory might be totally wrong but it would explain the 181 count and 'mirrored' aztec image.
http://i.imgur.com/z68P9Um.png(181 pixels to reconstruct = # of pixels in mirrored invalid aztec)
The way you have split it it is still invalid as the top right orientation marker is missing, and the first 5 codewords are still readable giving ".e: " (ignore the quotes) which doesn't work, and doesn't resemble the start of anything meaningful.