Actually the 24-letter alphabet lasted long after the 11th century. In “The Advancement of Learning” (1605) Francis Bacon uses 24 letters, with no J or U. He writes: “two Letters by five placeings will be sufficient for 32 Differences, much more for 24 which is the number of the Alphabet.” But still, I doubt the 24 blackbirds were meant to symbolize the alphabet. If you ask me, it’s just a nonsense rhyme with no particular significance.
Welcome to the Straight Dope Message Boards, sykerli, glad to have you with us.
When you start a thread, it is helpful to others if you provide a link to the Staff Report or Column that you’re commenting on. Saves lots of search time, makes it easier for other readers to get on the same page, etc. No biggie, you’ll know for next time.
This was a reply to a previous thread which discussed the origin of the rhyme. http://boards.straightdope.com/sdmb/showthread.php?t=6503&highlight=sixpence+song
The purveyor of the info was none-other than …ta da…C K Dexter Haven
To quote Iona and (the late) Peter Opie, editors of The Oxford Dictionary of Nursery Rhymes,