"et al": This is a Latin abbreviation meaning "and others." It's typically used in citations or references when there are more than two authors of a work and you don't want to list them all.
Authorship: In the world of literature, it's uncommon for multiple authors to use a single pen name like "Fitzwilliam." Collaboration does happen, but co-authors are usually credited by their individual names.
Therefore, if you encountered "Fitzwilliam et al" in a reference, it likely refers to a work with at least two authors whose last name might be Fitzwilliam (or a variation).
Here are some ways to find more information: