Thanks for your replies.
I wasn't that clear about where the tank was located.
The tank I saw was in the trunk of the car - not the back seat. And it was square with, if I remember the photo correctly now, there were tube coming out of it with another container type thing to the left (facing into the trunk from outside).
The 1971 Celicas, indeed, had the gas tanks (where you put gas in the car) behind the driver's license, but I've notice on some now that they don't have the license plate there and that most don't how the license plate at all. I would think they would only cover it over somehow if photoing it and didn't want it shown.
To be honest, when I owned the original 1971 Celica (bought it new) I never saw the care before I owned it and rarely after (in any paint color), and this was California where a lot of people buy new cars every couple years.