As I look into it more, Leeds seems less plausible than Brand. For one thing, Leeds died in 1908.
Franey didn't get into the restaurant trade in Paris until 1934. He could have met Barthe or heard stories.
From Chez Maxim's: secrets and recipes from the world's most famous restaurant, Presented by the Countess de Toulouse-Lautrec. 1962
>It was Louis Barthe, the former chef at Maxim's, who told me the story behind the Potage Billy By. In 1925, he was working in the kitchen at Ciro's, a restaurant in Deauville known for a special mussels dish with a particularly succulent juice. One day a very good customer, Mr. William Brand, decided to invite some American friends to Ciro's. Mussels are generally eaten with the fingers innFrance, using one double-shell as tongs to scoop the meat out of the others. As Mr. Brand wanted to spare his fiends this delicate operation, he requested that the juice be served without the mussels. It was such a success that during the days that followed, each of his guests returned separately to Ciro's and ordered the "Potage Billy Brand." For the sale of the discretion, it was placed on the menu as "Potage Billy B." and thus was born the "Potage Bill By" which has since become a classic of the French culinary tradition.
What's weird is, I can't find anything about Barthe, except that damned soup. I was beginning to wonder if he existed, until I found the Countess.