As a result of meticulous historical research using official batting, pitching and fielding statistics, boxscores, and the literature of the period, we have produced highly accurate seasons for every year from 1925 to 1970. (Deluxe Past Seasons run from 1971 to 2016)
For Classic Past Seasons, we don't have access to the pitch-by-pitch and play-by-play data that is available for current seasons. So we use other analytical techniques (using year-by-year and career data) and a review of boxscores and news accounts from the period. Left/right strategy remains a big part of the game, however. It's just that the left/right advantage is the same for every player, instead of being based on real-life differentials.
Some Classic Past Seasons (1925, 1926, 1934, 1946, 1948-49, 1954-55, 1961, 1966) also include a complete set of real-life player transactions and game-by-game starting lineups that you can use to recreate past seasons with even higher levels of accuracy.