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"20" (or "30") is not the street number since the building to the left of the plaque is number 2 and we found a number 20 on the other side, up the hill. The street is currently numbered 1-9 on the east side and 10-20 on the west side. Perhaps the street has been renumbered? It could have accommodated over 30 narrow-fronted buildings.

But, whatever the number signifies, we don't understand the function of these plaques - we've found another, very similar at College Place. Can anyone help?