The first docking of two objects in space was Gemini 8 docking to the Agena Target Vehicle.
Naturally, for two vehicles to dock, they need compatible docking connectors. They must be designed to attach to each other perfectly.
Careful manufacturing to standards can be enough to gain confidence that the systems will connect properly - today, every vehicle going to the ISS is designed to match its ports, despite never having a chance to do a physical fit test prior to launch.
For this first docking, were the physical flight units of the docking interfaces of the two vehicles ever fit-checked with each other on the ground, or did they rely on the "design, measure, and trust it" approach that is necessitated today?
