Factory Gretsch guitars come equipped with wooden “shims” under the pickups. This couples the pickup with the guitar body and can sometimes move the pickup too close to the strings, which is not desirable. You can replace these shims with compression rubber. You can order a set of compression rubber here or use computer mouse pad material. Filter’Tron style pickups are extremely sensitive, so having this compression rubber underneath allows for much more control over the height of the pickup.