So an O-ring got put into an invisible location -- worse, quality control did not catch it.
It looks like there was a rush put on to turn out 54 MLG assemblies, perhaps a bunch of overtime -- and one came out misassembled.
I'm guessing that SAS mechanics would not be inside this part during installation, but can't say for sure.
Next question: After changing over major MLG components, would you not put the a/c on jacks and cycle the gear?
How many flights did the accident a/c accomplish after the MLG changeover?