Michael Hamm writes:
Some sort of clip came with the car seat, but is meant (according to the
car-seat instructions) to be affixed to the seatbelt near the latter's
buckle, so as to prevent the seatbelt from moving as follows:
{The slack in the shoulder part moves down to become part of the lap
part.}
However this doesn't prevent more shoulder-part slack from appearing,
which is what I need.

No, you don't need that. It doesn't matter that the shoulder belt can
extend further, because what's holding the carseat in place is the lap
portion of the belt. Your locking clip is working exactly as intended.

I suppose you could add a gadget to keep the belt from extending
further, but it's really not needed for safe installation, which is why
it's not provided.

David desJardins