Well I am very happy to say that I have finally discovered in what issue of Seventeen magazine that the 'portrait of the girl' I've been searching for is in. It was a long time ago so my mind did play a few tricks on me, namely that I was so sure it was in a Seventeen from 1970 or 1971.
It actually turns out to be in a Seventeen from 1973. It's the May issue from that year and it has Deborah Raffin on the cover. The name of the feature that accompanies the portrait is listed on the cover of the Seventeen: A Romantic Story by the author of Jonathan Livingston Seagull.
The author's name is Richard Bach and the title (inside the magazine) of his story is: Letter to a girl who maybe never was.
Well she almost never was because I never thought I would find it! I felt such a sense of relief to have found this feature because I've been wondering about it for so many years - decades in fact. I wish I could send a scan to this post but I am unsure how to go about doing it - I can't upload a picture from my Library, where I have this portrait (someone who helped me locate the picture was kind enough to e-mail it to me).
I have recently bought this issue of Seventeen and I am expecting it to arrive next week - I can't wait - it will be like finding a piece to a puzzle from long ago. I just thought I would write to this post and give an update on my 'find'. Thanks for listening!