TOKYO — When President Obama made a historic visit to Hiroshima in May, there was no question that he was the first sitting American president to do so. But as Prime Minister Shinzo Abe prepares to pay his respects at Pearl Harbor on Tuesday, Japanese officials are scrambling to identify what, exactly, is unprecedented about his reciprocal visit.

Mr. Abe announced this month that he would visit Pearl Harbor. Japan’s Foreign Ministry, in news briefings, indicated at the time that he would be the first sitting Japanese prime minister to visit Pearl Harbor, the site of the surprise attack on a United States naval base 75 years ago.

It turns out, though, that he might not be the first, or even the second. It now appears that he is the fourth.

A few days after Mr. Abe announced his visit, news reports emerged that a predecessor, Shigeru Yoshida, had stopped in Hawaii in 1951 on his way home from signing a treaty in San Francisco and had paid a quiet visit to Pearl Harbor.