I really need to watch the episode again, but I am leaning to pre-Hartnell theory. Her mannerisms were similar to the Dr.1, and she had an impatient, berating nature to her, like Dr.1. The interior of her TARDIS was also reminescent of the early versions. Now, we know Dr.1 stole that TARDIS, but it was in repair bay and the chameleon circuit failure could have been already in effect at the time, and manifested as the last disguise it used when he initialised it. So, having used that TARDIS before could have been a contributing factor in him deciding to steal that particular one. There have also been hints to an earlier regeneration cycle, though those are rather vague.
But, all this together, makes me think it is possible, even plausible that she is the last of the previous cycle.