| The stock rear sprocket incorporates the cush drives etc. What Renthal do is replace the centre, where the cush drives sit, with a separate unit which you then bolt the sprockets to.
Initially it costs you more, but once you've got the carrier the sprockets are about a 1/3 of the price of OE sprockets and you have a far wider range of sizes to choose from.
Another benefit is that most of the carriers have a retaining lip to prevent the cush drives backing out and mullering your hub casting.
IMO, the Renthal are the best in terms of quality, price and also maintaining good chain alignment.
All the carriers i've seen use either 520 or 525 on the same unit. Some don't give good chain alignment depending on which size you have fitted, which sjows up as heavy wearing to one side of the sprocket. | 
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