Thanks to Mark Ehlert for the link to other source material. I took a look, but found nothing further in the way of publications based on the named colloquium. Of course, I can simply establish the one in question. Perhaps the BM simply used the number 21st because, let's suppose that it is the 21st of various annual colloquia, regardless of the actual name of each one. But I'm guessing!
Sincerely - Ian
Cataloging and Metadata Services Librarian
George Mason University