You more or less have deduced this for yourself -
you state you went to Corfu a lot on BEA Airtours Comets at that time - in the early 70's
and I too think it is a Comet wing tip.
However KT's wings were painted red...Plot thickens
Dan Air Comet 4B wing from cabin in flight
The 3 Dan Air 727's were flying then in 1973 on their first summer from mainly LGW BHX and MAN but the round windows throw me --- although the wing tip
- was a maybe...
I don't know why some folk here think it is a tailplane - The Comets were angled upwards and the tips on KT's were painted black.
Even seated down the back in the last row I do not think you would see the tail.
Carvair or Metropolitan>? Do Pay attention at the back, Class....lol