Why is “a” necessary before "Mr. X" and "Mrs. Y"?

There is the following sentence in the scene a reputed lawyer, Thomas Cohen gives advice to 16-year old client William Kane, the son of deceased bank owner on the issue of inheritance of his father’s property in Jeffery Archer’s popular fiction, “Kane & Abel.”

“Mr. Cohen continued. ‘The answer to your second question is that you have no personal or legal obligation to Mr. Henry Osborne. Under the term of your father’s will, your mother is a trustee of his estate along with a Mr. Alan Lloyd and a Mrs. Millie Preston, your surviving godparents, until you come of age at twenty-one.’"

I wonder why it is a Mr. Alan Lloyd and a Mrs. Millie Preston. Here Mr. Alan Lloyd and Mrs. Millie Preston are too obvious to both parties concerned, and different from the case of being told by your secretary, "A phone from a Mr. Smith Brown.”

What significance is added to by prefixing ‘a’ to the name in this particular case?

Solution 1:

The indeterminate a here implies that the speaker - and probably the listener as well - do not actually know Mr. Alan Lloyd and Mrs. Millie Preston. They're just names on the will, not actual people they are familiar with. The use of the a here means that this person could be any Alan Lloyd out there, since there isn't a concrete person to refer to.

This is the same with the phone call situation. "A phone call from Mr. John Smith" is a specific John Smith that the receiver is assumed to be familiar with. "A phone call from a Mr. John Smith" is a call from a stranger who introduced himself as John Smith.

Solution 2:

When a name is preceded by the indefinite article in that way, it usually means that the person referred to is unknown to the participants in the conversation.