All 1st. gen 4Runners through 1985 were shipped this way and then if a rear seat was installed it was an option. They did this to beat the taxes that would have been charged if it were shipped with the rear seat. They stopped doing that for the 1986 model and were shipped with the seat.
Importing them w/o rear seats wouldn't have prevented them from being hit with taxes, there was(still is I believe) a 25% import tax on light trucks. However, if it was unclear whether or not they would be classified as trucks or as passenger cars w/the rear seats installed, sending them w/o the rear seats would ensure the 4-Runner wouldn't effect the total number of passenger cars they could import that year.
