If I look in Google Ngrams, I see that the transliteration "honbu", meaning HQ, basically didn't exist until 1964. But it didn't surpass "hombu" until 1976. I believe Modified Hepburn was introduced in the 1950s, but I am wondering when the Library of Congress adopted it as an ALA standard--in maybe the early 1970s? I can't find any dates for this information, though I have looked on the LOC website.
In the deepest corners of the Library of Congress website, you can find the Spring 1983 issue of the Cataloging Service Bulletin. On page 51, you can see the specification calling for the modified Hepburn system for romanization.
An excerpt of the paragraph:
The modified Hepburn system of romanization as employed in Kenkyusha's New Japanese-English Dictionary (3rd and later editions) is used. For the syllabic nasal, "n" is always used preceding "b," "m," and "p." Romanization for words of foreign origin follows the American National Standard system for the romanization of Japanese, e.g., ベトナム Betonamu; ヴェトナム Vetonamu.