Unknown coat of arms of about 1700

Hello at all!
I need your help in determining a coat of arms (see the picture). It is likely that it belongs to the family Dondi dell' Orologio, but I cannot find pictures of their coats. Can anybody help me in finding an evidence? Is there some lexicon or something like that, where I could find it?
What is the most astounding for me, is that there are letters on this coat. I didn't know it was usual to have letters on one's coat of arms. Does somebody know what they could mean?
Thank you for your help!
Jangir from Germany