I recall, getting back to the timeline, that there was an issue of either Superboy or Adventure back in the late '50's/early '60's where Superboy was transformed into a Supergirl by an alien woman (women were the dominant gender on her planet). Superboy was badmouthing girls and how they can't do anything boys could do, so he was taught a lesson..
This is from memory... I recall the art was definitely Sikela (Superboy looked like he was 8... when he was transformed into a girl, she looked like a fully developed 18 year old girl, but still looked and was young). The secret ID was Claire or Clara Kent "Clarks's cousin who came to visit"
Does this count as a Supergirl for the timeline?
The character was called Super-Sister, but probably doesn't count, considering that Clark wasn't really transformed. It was a dream, induced by the alien feminist. Strangely enough, this Superboy story was one of the first I ever read, in an old reprint "80-page giant" I came across. Weird stuff.