15 rounder under 1920's refereeing and rules. Who do you take? Moreso, how do you see this playing out? Could Flowers' science stifle Ketchel's clout? Work rate, stamina, speed? How would these factors play in the match?
The historians on this site will give you a more educated guess because they have problably studied Flowers more than I have...but I think in this match up, Flowers' speed and southpaw stance would trouble Ketchel greatly...but even with that said...Flowers' aggressive style could get him KO'd. I would pick Ketchel by KO.
Flowers didn't do well against big punchers. Plus he liked to get in there and mix/swarm. Not good when you're fighting a Ketchel. Ketchel by one-punch KO within 5, Flowers ahead at the time Stan lowers the boom.