The '69 Z/28 timing covers I've seen are dated like "4 69" for April, 1969, not in a Julian format; don't know if or when that dating format started or stopped.
Yep, have a timing cover off a 67 model year HP 327 and its stamped "3 67" (March, 67). All the ones I have seen are month and year. Seemed many were stamped and many were not!?!? But as Mark mentioned identifying the date code would be extremely difficult once on the car. Heck, I could not see one even with the timing cover off the car until I sanded the area to remove paint. Sheet metal stamping is not deep in nature.