Wonder if previous owner forgot to take the bolt out and reinstall the screw plug.
The screw plug
https://www.brattons.com/plug-for-va...-manifold.html
When you took off was pan, was/is there a large spring on the bottom of the pump?
When you put the pan on, the spring holds the pump up into the motor.
The spring
https://www.brattons.com/oil-pump-retainer-spring.html