Isaac Gibson called 49 Winchester to attention. This was last October, and Gibson was standing in the balcony of the Roundhouse, a historic rock hall in the Camden Town borough of London, which had opened nearly 200 years ago as a railway shed. An hour earlier, 49 Winchester played to a sold-out crowd in the 3,300-capacity room below. Wyatt Flores had kicked off the evening, and every member of 49 agreed this was the biggest night of the group’s decade-plus career. The show was part of a European run that capped two years of touring behind Leavin’ This Holler , 49 Winchester’s 2024 album. As the last of the fans filed out into a crisp, fall night and the cleaning staff swept cans and bottles off the floor, Gibson raised a glass. “This is not the end, it’s the beginning,” Gibson said, his voice cracking while his band and crew, and Flores, looked on.…