Harold “Wisconsin” Johnson is the answer to your question.
Actually got to meet him this past Sunday in Lake Geneva, WI at the “Dreams and Nightmares” Convention put on by the Wisconsin Historical society. Also got to meet Tom Wham, who is the original artist for many creatures in the original Monster Manual including the Beholder.
Wisconsin Johnson was hired to to be an editor of the original first edition books. His degree from Northwestern University is listed as a BS in Biology. What that doesn’t tell you is he minored in European and Eastern history. He has extensive knowledge of European, Middle Eastern, Sumerian and Asian mythologies. He used that knowledge to add to the AD&D compendium. He drove a lot of design and content in the 1e days at TSR as he was promoted very quickly after he was hired.
Also, there is a AD&D Monster Manual II that is even larger than the original.
As to why he is called “Wisconsin Johnson”. He loves to wear hats, especially at conventions so that people could find him. He settled on wearing a Fedora exactly like Indiana Jones wore. Someone even called him “Indiana Johnson”, but Harold stated emphatically that he was from Wisconsin. So the moniker Wisconsin Johnson was born. Yes, he was wearing his Fedora this past Sunday at the convention I attended.
You always were an asshole Gormon.