I have found this topic sort of intriguing today...here are some more data points showing the 68-69 stamping was the same (at least that is my personal conclusion).
First, there is
this old thread that the seller describes the tailgate as a '69 tailgate...has the same stampings as '68 gates in this thread.
Then there is also the tailgate in the attached pictures...69+ lower trim present on the tailgate with stampings also found on '68 tailgates.
This thread suggests/confirms that the stamping changed in 1970 (70-72 had same stampings).
So in conclusion it appears there were the three groups of varying tailgate stamps that break down as follows: '67 type, '68-'69 type, and '70-'72 type.