In the late 60s, the San Juan 21 was produced by the Clark boat company. It became apparent  that there was a niche in the boat market for a lightweight, trailer-based design that could be daysailed and used as a week-end cruiser. They decide to design a boat superior to others in this niche, the San Juan 21. This boat featured a bigger V-berth, an easily stepped righ, and a fully retractable swing keel and kickup rudder. In 1971, the Mark I model was produced in Seattle and an additional facility in New Bern, North Carolina. By 1974, the Mark II model was put into production and more than 2,700 San Juan 21s were subsequently produced by the late 1980s.
Most of the information that I have obtained about the boat has been from the Internet. Infact I bought our boat off the net. I was selling a  Zodica raft that I had, and stumbled across an ad for a San Juan 21. The price seemed great for a boat that sleeps four and comes with a trailer. The wife and I went out the next week and bought 4-Play from a young couple.