It's easy to get confused. Originally parts of the Mirage Drive were described with a combination of sailing and bicycling nomenclature and much of this language is still used. The "clew" is the nautical name of one of the three points of the sail, and it attaches to the clew "outhaul block" (the hollow plastic piece that snaps over the back part of the sprocket) to connect the back of the fin to the Drive.
The "sprocket guard" is a similar hollow plastic piece that snaps over the front part of the sprocket to protect the cogs and chain from potential damage by submerged objects. Upon close inspection you will see that the sprocket guard is held in place with screws while the clew outhaul is free to rotate to some degree and is secured by the clevis pin to the fin (formerly known as the sail).
So by now you also know how the "mast" got its name! Avast, matey -- you'll want to get a nautical dictionary to decipher such other occasionally used terms such as port, starboard, bow, stern, chine, sheet, painter, block, padeye, cleat etc. etc.