The pin is smaller in diameter in the middle, right.
The coils of the trigger return spring sort of set in that smaller diameter area and that's what holds the pin in place. There is always pressure on that spring (more when you pull the trigger, less when it's just resting) and that keeps the pin in place.
When I order parts, I order extra small stuff (other springs, pins, etc.) to have as spares and to make the shipping "more worth it."