Here is the letter from Portugal

Dear Sirs.
Walking around my town in Portugal I found on the top of a hill a very old house (Cent. XV-XVI ?) with a very strange door with a symbol on the top of it.
I'm sending the photos, I hope that will be clear to you.
I hope that someone could tell me what it means.
Thank you.
Sincerely,
P. S.

Comments from our readers:

In those centuries (medivel) the workers that build houses and monuments were obligated by the architects to inscript a symbol to identify the workers. They use to do that to identify those who make mistakes. If the person that toke this pics whatch very closely, he maybe will find more and diferent simbols in other stones. This simbols could be found in many monuments around in Portugal like Mosteiro dos Jerónimos and many castles. Now what i find weird in the pictures is the shape below the symbol. It reminds a bat or something like that and i do not know any entrences with that lines, but I'm not an expert in ancient art, I might be wrong about this point.
Best wishes
Bruno