What follows is a breakdown of the manner in which everybody who played for the 1971 Pirates was acquired. You’ll see it’s not terribly different from the way teams now are built, except no free agents. The bulk of the talent came up through the farm system — the Pirates had a great run with their system from the early 1960s well into the 1970s — but there were some key trades that especially built up the bullpen. The Pirates weren’t quite as active on the international front as you’d expect.
One other difference besides the lack of free agents: There was no draft until 1965.