I heard this as a joke when I was a small boy.
Some travelling salesman got together with a farmer's daughter and she turned up pregnant. He wrote a note to said salesman which read:

I send to you this token of the whip socket that was broken,
And the footprints on the dashboard upside down;
Since you left our little Nellie, she's been swelling in her belly,
And I think it's about time you dropped around.

And the salesman's answer:

I received your little token of the whip socket that was broken
And the footprints on the dashboard upside down
Since I met your little Venus, I've been dripping from my penis,
And I think we're 50/50 all around.

As you can see, this took place in the horse and buggy days. The song probably came after the joke.