Most of these guys didn't know one anothers last names unless we had gone to grade school with them and knew them from way back as kids. Many you only knew them by their nicknames.
Interestingly, Joe Pistone said that when Lefty Ruggiero asked him to be his best man for his wedding, he asked him (Donnie Brasco) what was his last name so he could write it on the wedding form. They have been close for 6 years and would speak on daily basis but Lefty didn't know and never care to know the last name of the guy he considers his best friend.
In the Mob,it's apparently not unusual to spend years working and socializing with guys without ever knowing their real names. If they offer ,that's fine,but it can be disrespectful to ask for a "real" name.
These guys are so paranoid that the first thing they think is that you must be a cop or a snitch. The mentality seems to be,if I wanted you to know my name,I would have told you.
