What does this possibly Catholic symbol mean?

I have this symbol over the entrance of my apartment. I have been told it is a Catholic symbol, and the owner of the flat is probably Catholic since it is the dominant faith here. Searching on Google has not brought much information. Any idea what it means? It is from Buenos Aires, Argentina and the diameter is 6 cm. It's made of some metal but not sure which one, I assume nothing precious otherwise anybody could take it.