According to Mr. Google, it's a dialectal variant of sig. = signore = "mister, lord" = Mr.
Estimada Débora, le invitamos a unirse a un grupo de Italia, y de esta manera poder plasmar la consulta allí para que ellos puedan traducirlo. y darle una respuesta mas especifica a su pregunta.
I just think it is odd that it is the only Birth Certificate of the hundreds I have gone through that had this before the name. The only thing I can think of is that their occupations are "landowner" and maybe it signified that they were upper class.