They where first produced in 1953. There where some changes in 1960-61. I believe they where in production into the very late 1960s.
Many parts are still available new for them. The plates are the same as many of the newer plate type planters.
Dean Bartels of Peosta Iowa is a whiz on them. HE could tell you exactly when an where about them. He also has a lot of parts for them. He is on here some times too.