Yes! I figure either past!Ruth and ... returned-to-the-70s!Sam probably have a talk about it, or Ruth's clever enough to figure it out on her own once Sam's old enough to look like that DI Sam Tyler she met way back when. Assuming it is time-travel after all, I think that fits the logic, and it makes me feel better also. :)

And thanks very much! Filling the gaps is what I'm aiming for here.