I recently found that I-PEX has some very explicit mating instructions.
I realize the method is provided for MHF I with LOCK, but I think it applies generally, and I have had great success with it.
I have been using the method described in the doc ever since my observations and experiments upon first running into these.
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~R~
72/73 de Rich NE1EE
The Dusty Key
On the banks of the Piscataqua