"Opossom Up A Gump Stump"

I'd always heard it as "Possum up a Gum Stump"; Google agrees with me 539 to 2. I wonder if this is a case of, whaddayacallit*, when you add a sound because it's easier to say, like the b in chimbley (or, if you don't like that one the b in hombre).

Help me out here, nuncle