So when
Harry Heilmann was claimed by the Cincinnati Reds off waivers in 1930, he homered in the 8 stadiums in the National League, adding to the 8 he had hit in as a Tiger in the American League. Doesn't sound so difficult to hit a home run in all 16 parks, yet it hadn't happened in the first 27 years that the American League was considered part of MLB along with the National League.
As pointed out, it goes to show how few people played in both Leagues back in the day.
A few other tidbits about Harry Heilmann:
1) He succeeded HOFer Sam "Wahoo" Crawford in right-field for the Tigers.
2) He held the Major League record of 134 hits on the road in 1925 until Ichiro Suzuki passed it in 2004 with 145.
3) He hit batted over .390 4x, leading the league each time. His .403 in 1923 was ten points ahead of Babe Ruth. Ted Williams' .406 in 1941 is the only .400 average in the American League since Heilmann.
4) His career batting average of .342 ranks in the top 20 and just .0005 points behind Babe Ruth.
5) Heilmann's nickname was "Slug"
6) Heilmann took public speaking classes and had to learn to read the ticker tape since radio announcers did not broadcast from away games in those days. He used his imagination to make the game interesting and build excitement for those away games, where messages he received might simply say, “single to left.”
7) During World War II, he traveled to the Middle East as part of a baseball group entertaining troops.