Metal covers used to shield Jazz bass bridges and bridge pickups. These were typically installed over the bridge unit and the rear 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!
Length (at longest): 125mm
Width (at widest): 133mm
Height (at highest): 18mm
Mounting hole distance: 121.5mm