This is 1 (one) RSD AE TC6.1 Spool Cup. You need two to complete a set.
This hard anodized aluminum spool cup is more durable and are also easier on blades than the standard plastic cups. It is made of super hard 7075-T6 aluminum and is finished in a hard anodizing, which improves life even further.
These are the Version 2, which allow for more dog bone plunge, which allow you to run narrower arm spacing with the RSD DCJ's installed. They have also been lightened a slight bit, without compromising strength.