Anyone know what the 3rd digit in a 1968 body stamping means.

Seems like:

first digit is the year= 8

Second digit is the plant=

Third digit is ????

The rest of the digits are the SO number that is on Data Tag and Buildsheet

Please correct me if wrong on any of the above

And please offer any insight on the third digit

Examples:

F5E2E4E8-DDBB-4BDE-A34A-3AF33700AC57.jpeg54584D06-905C-4936-919B-E1723C6EF263.jpeg81562BAF-6F7C-44A1-9708-0D1108161010.jpeg