Spring spacers originally used by GM to raise the cars as needed.

If somebody argues the point for rego etc.. the original GM number was 3789664.

They are are a way to raise marginally the corner or both sides where coil springs are used. 

This hard rubber open ended coil seat has the stopper at the end as shown, for use at the base of the coil.

Manufacturer states they are 3/8" which is about 9.5mm

Supplied in SINGLE UNITS .. you can use one or get more as needed. etc..