Like many Board contributors, I've watched the GF at least 100 times. I was never unsure about Clemenza speaking that line. The pattern of Tessio's and Clemenza's voices are significantly different as is their intonation, tonal quality, voice pitch, and accent.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
