Greb seemingly crammed an entire career into just the year 1919. He fought an incredible 45 times that year - 5 more fights than Ray Leonard had in his entire career. Although most of these fights were officially no-decisions, newspaper accounts credit him with winning all of the fights, the vast majority by decisive margins. His opposition was a who's-who of top fighters from MW through HW, including HoFers Battling Levinsky, Billy Miske, and Mike Gibbons, contenders Jeff Smith and Bill Brennan, famed Dempsey conqueror Willie Meehan, and Joe Chip, who had handed Greb his only KO loss years earlier. If the entire rest of Greb's career were removed from the record books, he would still be an ATG just based on this year alone.
That is a highly impressive record indeed. However, it would seem that a single fight back then can't have been as physically demanding as they have become later. It is impossible to see Ray Leonard operating on his best level nearly once a week, the whole year around, after truly grueling fights
It's incomprehensible to me how he could have gone 45-0 against that level of opposition. Armstrong holding three of the 8 world titles simultaneously and Greb's record in 1919 are maybe the two greatest feats in boxing history.