Jean Rousseau’s custom watch strap business is booming.

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In 2018, when Pierre-Louis Follet joined the team at the New York salon of the Paris-based leather-goods maker Jean Rousseau, the family-owned brand had been in the U.S. for just three years. Things were going well, but if headcount is any indication, the best was yet to come.

“We were three and we’re about to be 30,” Follet tells Robb…