I did a quick...well, tedious and not so quick -- comparison of Harper so far this season against Frank Howard's first 39 games in 1969, Killebrew's first 39 in 1959, and Roy Sievers' first 39 in 1957. We are watching a big-time season.
They are all close, but Harper beats them in each category except home runs. Killer had 15 homers by his 39th game while Hondo and Harper had "only" 14. All of them over 30 RBI's (an accomplishment for Roy, since that team could not hit worth beans). Harper's OPS is about 1.2, while Harmon's was 1.022; Roy was .959 and Hondo's was .931.
Where Harper stands high is his batting average of .338. Next closest was Roy Sievers at .297.
Statistically, Harper's hitting is a little above the most powerful hitters ever to play in Washington.
None of them could field like Harper, although Sievers was very smooth. He had wrecked his shoulder after his Rookie-of-the-Year season, and never threw well. Killebrew played third by subduing ground balls...knocked them down, off his chest, off his forearms. Fans were horrified after having watched Eddie Yost for years. Howard was a fine athlete with a great jump, all six-foo-seven of him. Might have been an all-star power forward in the NBA. Had no range, though.
(Note: can't get dependable stats for Josh Gibson. The Negro League played barn-storm games during mid-week, playing each other on weekends. Stats are shaky).