A further correction:

Lonzo and Oscar, whose real names were Lloyd George and Rollin Sullivan, recorded a hit version of "I'm My Own Grandpa[w]" in 1947, but they didn't write it. It was written by Moe Jaffe and Dwight Latham. They allegedly got the idea from a "Mark Twain anecdote" – but the anecdote hasn't been traced to my satisfaction.

See I'M MY OWN GRANDPA in DigiTrad. From there, you can follow links to other threads with more discussion.