I had asked about the mystery symbol in San Michele, on the Slow Travel Italy forum, and got the answer.
* It is the symbol of the Confraternita della Misericordia, which is a volunteer group that still nowadays helps out people in need.
Think about ambulances, most have the same symbol on it.
It means that the man (or woman) buried there was part of the volunteer group of the Misericordia
The two “Omegas” should symbolize the Bishops’ hats so it would mean that the fraternity was started by the bishop in the name of Christ – the long cross in the middle, less romantic but true.
You can find other similar symbols in other cemeteries, like Soffiano, near Florence:
I love the internet!
PS I just realised where I had seen that symbol before. On 7 April, I had attended an Ecumenical service at Chiesa di S. Giacometto di Rialto, and many of the men and women in the congregation were wearing cloaks with that insignia.