FYI. While it's been a while since the above posts I offer the following for what it's worth. Reference the discussion and pictures in the January 2013 posts. The posts about the small screws not attaching the two parts of the truck frame together.
I have found some Atlas O 3 rail trucks where both parts are threaded and so the screws DO attach the two parts.
And as shown above, I have also found 3 rail trucks where only the "top" part is threaded and so the result agrees with the pictures and words above.
To investigate further I examined a truck from an Atlas O car that came from the factory with 2 rail wheelsets and therefore without the big black coupler adapter bar as shown above. This car has both parts of the frame threaded and has the small screws installed. So, the two parts are connected and truck is rigid from the factory.