I just installed my new Trico Exact Fit blades. As usual with these things (because there are different models and different mountings), I'm lost until I look it up online. The 18" blade had a little "OPEN" legend on it, and I could not see how to open it until I unsnapped the little housing and saw the "OPEN" hatch was indeed that, a hatch. I used a mini-screwdriver and got it open, did the same thing on the old wiper, clicked it on, and Robert was your mother's brother.
The 26" blade did not have the same kind of assembly. A moment's study showed me that the big wiper just snapped out, and the new one went right in. I tested the wipers on the newly cleaned and clayed glass. Smooth and silent.
Is that your experience too, that the different-sized arms have different mountings?