Silber's statement about the song going back to 14th century England is based on a series of unsubstantiated speculations, as noted by Malcolm.
The 'folk' versions found by Silber just reproduce verses remembered from published versions. They only emphasize the popularity of the stage-vaudeville character.

Even more popular (based on verses often reproduced- see Belden's Ballads excerpt above) than Reeve's 1836 version was the one titled "Billy Barlow, a Favorite Comic Song as sung with unbounded applause at the Western and Southern theatres, by Himself." Arranged by P. F. Fallon and also published in 1836 (in Levy Collection). No sense in reproducing the entire song, but here are a few verses that were poorly remembered in the version from Belden, given above by Joe:

3. They say there's been a robbery committed in town,
I don't know who could have done it but Canker the clown
I feel perfectly safe I'd have you to know
I should like to see a man try to rob Billy Barlow.

7. They say there's a wild Beast show come to town,
Of Lions and Monkeys and Porcupines too
But if they start to show I'll beat them I know
For they ain't got a varmint like Billy Barlow.

8. They tried to buy me to go with the show,
But the Monkeys got jealous & the Lion snapped at me too-\The Hyaena growled, and looked at me so
Thinks I 'twill never do for you Mr. Billy Barlow.

11. The Tailors in town etc., etc.

One verse (10), There's been a nigger here asking about a long tail Blue, about a long tail coat in a color popular with minstrels, has led to some confusion with Blue Tail Fly, through the folkification process.