This is the factory Glock 28 Newton (6.29 pound) extra power striker spring. Primarily used overseas by military and police with ammunition that has exceptionally hard primers.
Also very popular with Canik owners when combined with the CZ reduced power plunger spring to reduce trigger pull weight. Even though the Glock striker spring is an extra power spring, it is a reduced power spring compared to the factory Canik spring. A polish of the moving action components and replacement of the striker and plunger spring usually drops the trigger pull on a Canik down to around 3.5 pounds.
It's an expensive spring in comparison to most Glock springs, but extremely hard to find. We're still trying to source them at a better price.