It was supplied by Sony to solve the problem found on this model of white noise on the picture which is caused by static build up on the head drum. The guide sits over the drum and earths the central spindle.
Note, older SL-C9s may experience static pick up due to dry joints in the
head pre-amplifier module. This is the metal can which pivots back giving
access to the head drum pictured.
If your model is suffering from this earthing problems then you will need to order a kit.
See also the SL-HF950 static discharge notes