David,
Unfortunately, neither have I. Like you, I thought this was something that started with the 69 Model Year Series 2s. The text I quoted with respect to post-1967 cars having this tag was from Haddock and Mueller’s Originality Guide, and implied to me that they had seen Series 1.5s with the tag. Having just searched through detailed photos of 6 S1.5s sold on BaT, none of which appear to have the tag, I have to think that Haddock & Mueller may have got it wrong, unless there is some other explanation. This would not be the first such error I’ve found in this book, unfortunately. I guess I set my expectations too high…
The Jxx tags have been seen as late as J70 according to this thread, with specific cars being listed for J69 and J70.