D.B. Cooper
True story. A former postdoc of mine was an immigrant from Serbia. His first name was Andrej and he had no middle name. When he became as US citizen, they offered him the possibility to change his name. He wanted to make it Andre J. They wouldn’t allow it. When he brought up Harry S Truman, they still wouldn’t allow, but they did decide that if he knew that much American history, they would not bother to quiz him on it. He now uses only Andre, no middle initial or name.
Wondering if the OP is being read. I am not asking for people that have only initials for their name, asking about adding the “ONLY” bit.
Google turned up info that actual military (USAF, specifically) use would be (IO) rather than (only).
That was snitched fromHenry Cho, who I’m sure snitched it from someone else.
It is, in both senses of the word, Schrödinger’s initial.