The floor springs in the Conference Hall (City Hall) are less ornate (without the engraving) than the double action ones that can be found in the Old Council House.
According to this book: London's Industrial Heritage By Geoff Marshall, the idea for the "Slave" came about because of a bad back and a rattling door in 1882, finally being patented in 1890.
The above patent date (if correct) mean the "Slave" could not have been put in the ground when the Old Council House was first built but could still be the originals to the building.
James Adams & Son were still trading in 1921 according to this site http://pubshistory.com/streets/UnionStreetSE1.shtml and patents were still being issued as late as the 1940's which relate to this door spring http://www.patentmaps.com/assignee/james_adams_%26_son_1.html and others.
Below are the shots of a single action "Slave" incorporating new spring.