That is rather awkward. By saying the "Main Street Gym burned down in 1951 and was replaced
almost immediately by the Moose Gym" that implies the Moose Gym began in 1951? I think what the original author is intending to connote is that the folk who frequented the Main St gym, in its newfound absence, immediately made the Moose Gym their new home. (And then those Main St habitués apparently moved back to Main St when a new gym, across the street from the former one, opened.)
The Moose Gym had a long pre-1951 history. After the Moose installed their officers at 300 S Olive in December 1926, they began having boxing there in pretty short order -- there are, for example, a number of notices in the 1931 papers about how the Southern Pacific Association staged amateur matches as eliminations for the '32 Olympic boxing team. In 1951 the World Lightweight King, Jimmy Carter, boxed four rounds there to great acclaim. The Moose Gym/Royal Apartments fell in the fall of '62.
Nathan