I am sure any connection with scouting would have the Scout emblem. As the Board (plaque) has a simple wreath I think most large organizations could be ruled out. Mainstream churches most likely would have had a cross symbol rather than the simple wreath. Smaller or less ostentatious denominations might favour a simple wreath of course.
As the Memorial Board (plaque) was not considered important - I suggest - I get the impression that the building that housed it was not that important to those demolishing or re-developing the area. Moreover, anyone using the place should have been aware of the Board, unless, of course, it was situated in a closed off or rarely used part of the building in which it was originally affixed. Another point might be that the original group, society or whatever had ceased to exist soon after WW2. This might explain the single name Mason added to the Board