In an email, I was asked this by a reader:
“I was reading your article about how the change to allow a 26th player starting next season and then only to be able to add two more players in September, instead of up to 40 (which you posited could be detrimental to prospects), and it occurred to me that MLB teams were allowed for a number of seasons in my following things (say 1960 until some time in the 80’s) to have 28 players for the 1st month of the season and then cut down to 25 around May 15th (when the season started later in April). I tried to find a discussion on one of the forums on your site that addressed your article so I could ask the question in the forum, but did not see a forum discussion of that article. Do you remember such a period when teams were allowed to start the season with 28 players for the first month? If so, do you know when that was?”
The answer is from 1957 through 1967, teams could open with 28 players but had to cut to 25 by 30 days later. In 1968, they began the rules still used today.