Googling for Bowler and Earl of Derby brought the answer that while Bowler designed the hat, the Earl of Derby was the guy that wore it.

While looking for this gen I found a wonderful site that gives a vast quantity of names of hats and also lots of fascinating information on them. It is the data provided to ‘4th Grade Interdisciplinary Unit’(?) students to enable them to take a test. Do take a look at it; a mind boggling collation!

http://www.prof-dev.okcps.k12.ok.us/coreunits/4hats.htm

It confirmed what I suspected but hesitated to say originally; that it is surprising how many names for men's hat styles there are and how few for women's. I wonder why that is. Must be a Mars and Venus thing to do with a desire to categorise and label!

Edit: thanks Bean, that's an interesting site. I shall enjoy browsing it and its links.