Good question.
Thew 70/100s had the serial number in 2 different places.
Here is where the serial number is on my 70:
The 70 was made in Feburary of 1965.
My 100 has the serial number on the reduction housing.
The 100 was made in June of 1964.
There is a possibility that the rear end did not start it's life in a 70 or a 100.