IIRC the seat belt locks when there is a rapid change in direction; the mechanism that locks when it detects rapid deceleration or a sudden jerk that would occur in a collision. For a child's seat, I am not so sure, but I would assume that they tested it with modern seat belts that lock when you simply pull on them a little too hard.
I would call the seat manufacturer and ask them what to do, and if it would still be safe for use.
Red 1988 GT under restoration!
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