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There's no hidden latch on the seat, only an inertia latch, installed in the hinge itself.
This is actually a pretty common problem. I have the same problem with my 81. The drivers side seat back has never locked up, but the passenger side one has done it occasionally, almost since the car was new.
The inertia latch is in the outside hinge of the drivers side seat. I've found that by sliding a long thin screw driver into the hinge, you can release the latch. I forget exactly where you release it and I can't get to my 81 at the moment, but if you do a Forum search, you should find some help with the latch, as it's been discussed in the past.