Thank you very much Jack and CarolW for your replies to my post. I hope this message goes to both of you – I'm rather rusty.
I don't know if "Uncle Abe" was a family friend and not a blood relative, but what you say is relevant, CarolW. I used to introduce family friends to my small children as "uncle" or "aunt" and one day, out of habit, I called their father "uncle" ! Oops.
And, thanks Jack for looking up all those sources for me. It remains a mystery.
Jamie H