In the late summer of 1969, as the New York Mets began their improbable run toward underdog immortality in the World Series against the heavily favored Orioles, Tom Seaver and company played a four-game series against the Pirates at Pittsburgh's Forbes Field. LIFE photographer Co Rentmeester was there—just days after the Mets had moved into first place for the first time in the team's history by beating the Expos in Montreal—and, as seen in these never-before-published pictures, Rentmeester captured the team's endearing mix of intensity and casual, low-key confidence just weeks before the playoffs. Pictured: Seaver pitches to future Hall of Famer Willie Stargell.