When SUPER DC GIANT S-34 reprinted "The Girl Who was Supergirl's Double," there was an interesting change in the caption for the splash panel.
The original reads, "Phantom Girl, Wonder Woman, Saturn Girl, Power Girl and Mighty Maid! Yes, readers, you've all read of these feminine marvels time and again!" The last two, "Power Girl and Mighty Maid," were actually just Lois Lane (Power Girl) and Supergirl (Mighty Maid) and neither was for real or beyond one story.
This caption was changed for the reprint to read, "Phantom Girl, Wonder Woman, Saturn Girl, Wonder Girl and Black Canary! Yes, readers, you've all read of these feminine marvels time and again!" Those last two, "Wonder Girl and Black Canary," were known in 1971, but not so much in 1963--Wonder Girl was still the younger Wonder Woman and Black Canary had yet to return from obscurity on Earth-Two.
When Lena wears a Supergirl outfit to a costume ball, Dick Malverne is convinced that she's the Girl of Steel and he can't be persuaded otherwise. In the end, Lena tells Dick that he's right, which leads into the next issue and "The Forbidden Weapons of Krypton."
In fact, when she said she was Supergirl, Lena was being thought controlled by Lesla-Lar from her secret laboratory in the bottle city of Kandor. The evil blonde then uses her exchange ray to trade places with Lena Thorul--leaving Lena with false memories of being Lesla-Lar.