My guess on a 7/16 unf thread was a pretty well educated one. My old job was doing Australian compliance modifications to japanese imports. This included replacing seatbelts in EVERY car with new ones. It is law that any retractable inertia seatbelt be mounted with a 7/16 unf bolt. Just as a useless tidbit you'll probably find that the child restraint anchorages are imperial thread too (5/16 UNC).

The 1988 Hilux surfs (4runners) we used to import from japan had NO rollcage in them at all. We had to fit them. The japanese don't really seem to care about their own safety when building JDM vehicles.