Update: Stupid, stupid Ani! (Hits self on head). You asked everyone in Calcutta about this damned mark, except for the person who was sure to know. And, as expected, Mr. Souvik Dutta - resident expert on all matters Hindu - has given us the answer. And here it is.
This is a Shiva-Shakti mark. The three lines represent the 3 gunas - Satwick, Rajasik and Tamasik or could represent the 3 worlds. The circle represents the representation of the Motherly "teep" (bindi). How could you forget that Banga was a matrikendrik samaj?
How indeed? It seems the matriarchal conspiracy runs deeper than was previously thought. They put a damn sign on every single wall, that literally screams "Mummy's got her eye on you". Could they BE any more devious.
Perhaps a little more information is in order.
The above motif is everywhere in Calcutta. More precisely, every residential building that was built before independence has this motif, or a variant thereof, etched in relief on at least one part of an outer wall.
Here are some variants that I have noticed (there are many more, I am sure - but they don't come to mind).
I have asked my grandparents and others of their generation, but nobody has yet been able to tell me. Is it some kind of good-luck charm? Does it ward off the evil eye? Is it some obscure Vaastu Shaastra mark?
Two things strike me about this motif. One is that it seems to be a Bengali tradition. Its markedly absent from buildings in any other city I have visited or lived in, but I have seen it in some semi-rural areas of Bengal. Secondly, it is markedly absent from all recent constructions.
This would suggest that placing the mark on a building is some sort of Bengali tradition that we seem to be missing out on. If that is the case, then its yet another piece of history that we are losing through ignorance. Alternatively, it could be a passing fad whose time has passed.
Either way, I would be most grateful if someone could tell me - WHAT IS IT?