These locking plates are located between the joint bolts and hold two bolts together, preventing them from moving in any direction. If you don't use locking plates and a bolt comes loose, the entire drive shaft will soon be loose, with all the consequences that entails.
Homokinets only apply if your car is equipped with an independent rear suspension (IRS). If you're not sure which rear axle is mounted, check to see if you have rubber boots on both the wheel and transmission sides, then you're dealing with intermediate axles or IRS. If they are only present on the gearbox side, they are swing axles. The only exception to this rule is the Syncro Vanagon, which also has front driveshafts with homicaps.
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