Hext if you mean a stiffened Derby, they go back to the 1830's in Jolly ol' England. They were named after, but most likey not invented by Edward Stanley 12th Earl of Derby, Derby being a town in the county of Derbyshire.
I think Dave's hat if I remember right is a Bailey Sargosa, that's called a Western Derby because its got a larger brim that the Bowler or English Derby like the one Wll wears. That Bailey is a nice hat for the right persona, I try them on a lot of folks when puttin' together an outfit. I often like the look better than they do, but I sell them kinda steady.
One thing, you might want to do some tryin' on and lookin' before you buy. Not every one looks as fine as these to gentlemen in one. I end up lookin' like Teaspoon in The Young Riders in one. I've been tryin' to think how you'd look in one. Can't say fer sure since I don't have yer carcass to try stuff with. Let me know how it works out and post a pic if ya do it.