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The plaques are on the stone pillars either side of the entrance; plaque 1 to the left in our photo and plaque 2 to the right.  The strange arch is actually part of the cutting edge of the tunnelling shield that was used to dig the tunnel.  There is an almost identical pair of plaques at the northern entrance to the tunnel.

A third abandoned tunnelling shield can be seen and walked through in a tunnel at Bank tube station.