The cleats are used for the outhaul, flattening reef and deep reef. All three lines are lead internal, inside the boom.
The early Catalina 30 also used a clam cleat for their outhaul. It was located near the gooseneck on the underside of the boom. We are not certain (although it is likely) that the hole spacing on this new cleat is the same on all of the old cleats. You might have to drill one new hole.
The original plastic cleats tend to flex under load and release the line. The plastic also wears rapidly and soon wont hold at all. Aluminum is much stronger and won't flex under load. It also includes a roller to reduce friction when you haul on the line at less then a perfect angle.
Fits line size: