I assume, to be sand blasted. Quite a chunk of cast iron. The Concordia Yawls were not supposed to be expensive 'yachts' in their day(if I recall their history correctly). The cast iron instead of lead must have saved some money in building?
The builders of those boats might not believe that today they'd have a following of owners that lavish much $ and care into them. A lot like the Hinckley aura, only in wood. A few sailboat designs are proving to be timeless.