Metal covers used to shield Precision bass neck pickups. These were typically installed directly above the pickup on vintage style Precision basses. These were designed to act as a shield to RF interference noise for the pickup, making it operate more quietly.
Newer models of Precision basses typically omit the cover due to modern wiring typically being less disruptive to pickups. But there are still some players who enjoy the look and feel of the cover, nevertheless!
Length (at longest): 128mm
Height (at highest): 22mm
Mounting hole distance: 118.5mm