Metal covers used to shield Jazz bass neck pickups. These were typically installed directly above the neck pickup on vintage style Jazz basses. These were designed to act as a shield to RF interference noise for the pickup, making it operate more quietly.
Newer models of Jazz 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!
Width (at widest): 52mm
Height (at highest): 23mm
Mounting hole distance: 122mm