First of all, I assume that you’re talking about the Singer Class 66 machines, not all Singer machines.
In my experience with the Singer Class 66, the handcrank “boss” began appearing in 1913. I don’t think that they were designed as a “motor boss”, but later on when electrification began, the motor bracket was designed to fit onto the handcrank boss, or “hand attachment boss”.
I wouldn’t expect a 1910 Singer 66 to have a boss on it.
CD in Oklahoma