Monitor Series 2, M3 9173340 bracket question.

These speakers were stored in the attic for a long time. I have the brackets and speakers but can't quite remember how to lock the bracket in a given position. It is not the type that is splined with a flat spot. There must have been more than one type of bracket mechanism. Polk didn't really have an answer. They just told me that the speakers are 20 years old. I have pictures if that will help.
Thanks!
Thanks!
Answers
Just another view.
Thank you for any input!