Platter Damping Rings
First used on the Dunlop Systemdeck and described as a peripheral wave trap the simple rubber ring was really just a gimick. However, the rubber damping ring is most useful on turntables such as the Garrard or Lenco where there is a rubber drive wheel acting on the rim or underside of the platter. Damping rings are employed today on new Garrard turntables which proves their worth as a sound reduction feature. Cast Aluminium platters will ring like a bell if undamped and the drive wheel provides a constant excitation which is audible.
For the strobed Garrard turntable two NBR “O” rings of 5.33mm thickness are recommended as is used I believe in the 501.
For plain platter turntables such as early Garrards or Lenco's up to four rings of 3.53mm thickness can be employed.
Many Lenco enthusiasts upgrade their turntables with a "Peters Top Plate conversion which adds rigidity to the turntable. It also lifts the platter above the chassis thus enabling the fitting of damping rings. The standard Platter on a GL75 has a recess of 17mm which is enough for fitting 3 of the thicker rings or 4 of the thin ones. Choice is yours and depends upon which you think looks best. The photos below show both on an experimental platter with strobe markings from a GL75
I no longer supply these rings direct but they are available form Polymax here in the U.K. https://www.polymax.co.uk/o-rings/rubber-nbr-oring/