What does 4 and 20 b b b in a p mean?

4 and 20 Black Birds in A pie Swazoo

Black birds baked in a pie.

4 and 20 blackbirds baked in a pie schwazoo

True. Sing a song of sixpence,a pocketful of rye.Four and twenty BlackBirds,Baked in a Pie.

Mary had a little lamb.