They way it was explained to me is,
The first number designated INTERNATIONAL
The next 2 digits is the length.
The next letter designates the bed configuration T for Twins
D for Double bed.
The next digit is the year model
the next letter is where it was built S - California
J is for OHIO
The last 4 number are the sequencial assigned from factory line
This is how a 1972 VIN reads.
I do not know if it is different for the 60's models,which I believe you have . Someone out there on the forum may know.