Old time guys like Benny Leonard and Jack Dempsey and, I think, just about everybody at that time did a lot of calisthenics. They valued flexibility pretty highly. Nat Fleischer- I'm almost positive- wrote a book on boxing conditioning, based on the workout routines of Leonard (Benny) and Dempsey. It featured the daily calisthenics that these guys did, something like 20 minutes in the morning and another 20 in the afternoon.