The "double black box" was first used in early 1974. The "single black box" dates to 1972(?) or thereabouts. No box at all would be late 60s or very early 70s, but usable into the later 70s. The "single box" was used mostly on new or rebuilt cars, and the "double box" wasn't universal for a few years after 1974, as it took a while to put them on all of the older cars.
If you can read the service date, usually to the right of the LT WT stencil, that's a better guide, but that usually requires having the car in hand OR a cooperative manufacturer who posts the information.
If the car has a LD LMT and LT WT, but no CAPY, it's probably 90s or later, as the CAPY line was eliminated sometime around then. Does anyone have a date for that change?