I am not sure, and can't wait for others to weight in but from my experience they have been mostly 60s pieces... I think i may have had 1 or 2 70s pieces... but 60s comes to mind and early 70s.... but I suppose I just have never noticed,,, and the ones from the 60s feel more like cardboard/stiff canvas and this one looks more pliable so if you are using for dating you need to look at other labels and this one looks later/newer than than the 60s
here are some labels... the top is mid 60s.... the middle is a 50s/60s cusper and the bottom is a 60s/70s cusper
hope that helps