The thumb latch does nothing with reguard to holding the catcher on.
The catcher hangs on the back of the base. So it needs to sit right.
It should just sit there once you hang it, push the mower about here there and everywhere and it shouldnt fall off.
The thumb latch holds the door up.
On those older mustangs, looking in opening at the front of the catcher on the left theres a black plastic thing with a lip on the bottom that Ty said about.
You need to hold the door up so it slides in over the top of the black bit, this will hold the door up pretty much 90% open. Then lift the door the last bit with the lever and it will lock in the thumb latch.
As grumpy said, its a bit of a sideways jiggle to get the thing on right.
Looking at the picture its the right catcher.
The later catchers have an opening thats a bit shorter and dont quite sit right on the back of the mower.
Sit the catcher on, then using the side lever lift the flap up and it should lock into the thumb catch by itself.
On those older ones the rear door is longer with the spring loaded flap on the bottom, so the plastic bit in the catcher is two fold, the catcher wont hang with the door down, and when you start taking the catcher off the black plastic thing holds the door up so it doesnt drag grass out with it.
