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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 southern entrance to the tunnel.  The difference being that the texts about the cutting edges makes it clear that one tunnelling shield was used for the tunnel south of and under the river, while another one was used for the tunnel north of the river.  We are not even sure that fact is interesting enough for an extremely nerdy London quiz.

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