The problem only came to light when I found that the torso with the back pack attached didn't look centered. It was then that I discovered the X-indentation marking the center of the butt plate wasn't.
Who would have thought that a tiny amount of asymmetry could be so troubling?
However, I'm pretty pleased with the repose on the model above has turned out.
Below, you can see I managed to break the bazooka casting. Fortunately, this is from the current 1/144th scale range, so I have spares in case anything worse happens.
For those who might ask why use the smaller bazooka? Have you seen the size of the originals: they're ginormous–totally over-the-top humongous weapons.
So, I only managed to get this far with assembling these two mechs before I had to call it a day, as supper was being served.