Comparison of Tianzhou-2 and Tianzhou-3. The green star points to a hinge like device at 0 degree. All direction is clockwise.
Tianzhou-2:
- There is a big round plate on top of a cylindrical structure right next to the green marker.
- Further next is the attaching port for robotic arm.
Tianzhou-3:
- The plate is gone.
- In the place of robotic arm attachment port, it is a cuboid structure.
Regarding the attachment port, I suspect that it is a modification. Tianzhou-2's port is at an angle in between Tianzhou-2 and the node module (节点舱). It is difficult to maneuver the robotic arm's attaching head to reach the port. So Tianzhou-3 tilts the port to radial direction.