Wiper inserts for Mazda wagon rear blade

The rear wiper blade on a Mazda 6 wagon is a custom 12″ arm, so it can’t be replaced using just any 12″ wiper replacement.

I remember in the old days we would buy just the rubber insert and not routinely replace the wiper arms, so I bought a “6mm universal insert” but that didn’t fit the metal supports on the rear arm either.

What are the different types of replacement inserts, and how can I tell which will fit a particular wiper arm?