That sheds some light on things.. So 68 springs no year designation?
Depends on the supplier it appears. Have some with a number (year) as the last digit in the bottom line and on others no number (year) as the last stamp
There are some that were installed in 68 (no Shelbys that I've found) where the supplier used a pattern such as plant followed by a dash and then a single numbered year followed by month it was stamped. Example G1-8C. So those are not day specific as the other supplier would be
Seems like there were a bunch of suppliers but it also depended on which plant used which supplier on a regular basis