BR explained in 2010 how it calculated pre-1975 saves:
https://www.baseball-reference.com/b...ives/7371.html
"For 1920-1949, saves are awarded on the "encyclopedia basis": a pitcher who finished a game his team won, but did not get the win himself, is awarded a save. This policy is so named because it was used by the first Big Mac encyclopedia and later by Pete Palmer.
For 1950-1968, the policy is to use the 1969 rule (the first official one) and apply it retroactively. However, David Smith of Retrosheet admits that it hasn't been applied as rigorously to these seasons as the encyclopedia rule was applied to 1920-49, so there may be instances where saves were or were not awarded precisely according to the 1969 rule.
For post-1968 seasons, the rule in place at the time is used."
I presume this hasn't fundamentally changed.
The 1969 rule says: "The relief pitcher must enter the game with a lead and hold it. He can be taken out of the game only for a PH or PR. If more than one pitcher qualifies, the scorer gives it to the 'most effective' one."
The 1974 rule says: "The relief pitcher must face the potential tying or winning run at the plate or on base, or pitch effectively for at least three innings, and in either case preserve the lead. Same as #3 above, one save per game, given to the most "effective" reliever."
The save rule beginning in 1975 is the one we all know and love today.
The explanation doesn't address pre-1920 saves, but presumably they calculate them the same as 1920-49.