I'm not sure what it's called, it came on the car when I bought it....
It took me years to find my 68 340 fastback Formula S. I found this one in Tampa, FL and drove it back to Detroit back in '88...
the PO had the axle rebuild by Dan Dvorak with heavy duty parts... 8 3/4 3.91 sg...
We used to have fun with it at the Woodward Dream Cruise each year... the first three years, the cops didn't care if you did burnouts... I could get the tires spinning for as long as I wanted... The crowd loved it...