That was the Exhibition Hall, near the present-day
Forum site. Destroyed in the Halifax explosion, it was replaced by a much more pedestrian looking Industrial Building that was demolished when the Post Office complex was built in the 1970s. I remember being in it in the 1960s and being alarmed by the bouncy, creaky floors. Courtesy of Dmajackson:
Source: thechronicleherald.ca
"In January 1976, onlookers watch as Halifax’s Industrial Building is demolished to make way for a new $15 million postal station on Almon Street in Halifax. The building opened in 1927 as a display hall for the annual Nova Scotia Provincial Exhibition, also serving as a military barracks, a vocation school, the city market and Atlantic Winter Fair displays"