Seat Adjustment Problem on 107

The driver's seat on my 380sl won't slides back and forth, but won't stay in place anywhere on its rails. And I can't figure out how the
locking mechanism should work.
I can see that lifting the handle coming out from under the seat next to the console activates a lever under the seat's other side (via a cable), and that the lever then raises and lowers a plate further back. However, I can't see how or where that plate is supposed to lock the seat in place, and why it isn't doing it. So, any help would be appreciated.
Thanks in advance.
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