It is easy to check before you install it by sliding it over the bb, to make sure it fits, and measuring the width when bottomed out to see how it fits the shell width.
In all honesty, the left (NDS) retainer doesn’t do a whole lot. I imagine there is some minimal support provided to the cartridge; although minimal is being somewhat generous when talking about a plastic retainer.
The best thing it does is help keep crud out of the bb shell and the threads clean.
John