Although Adolphe Singer was the president of the ACO he was never a member of the ACO and was therefore not allocated a membership number. Whether being president meant that he was automatically a member I do not know as he is not listed. I dont think anyone will ever answer this question unless they have a copy of the official ACO annuaire from the early 1900's I will therefore answer the question and put the millions of people out of their misery.
And the answer is....................
Albert Arnauld a lowly pharmacist who lived at 72 Avenue Thiers, Le Mans, Sarthe. He just happened to be the first individual to join the ACO and was issued with the prestigious membership number one. You would have thought that the ACO would have allocated it to some notable person but like today the ACO do things their way. As in the allocation of tickets you never know what you might get when you pay your money.