The author of the book was a former City Archivist. Is there a list of sources inside? Ask Local Studies about it.
You tend to get groups of people who came out to be photographed when official surveys were being made prior to demolition, but not such a large and organised group, and the photos would be focused on the defective buildings, not the street. This looks more like an organised event, possibly as AlanBoyd suggests, a farewell gathering.