The ball and spring held in the pump body with the pin are the by pass valve. They limit the filter pressure to allow oil to flow around the filter canister if it becomes clogged. They also keep the canister from popping from too much pressure.
Changing the filter assures that this device never comes into use. I once saw a ZTU filter can totally filled with dirt and bearing frappe. The owner never knew a filter existed down there. The only passage open was the bypass hole to the intake.