Here are the corresponding parts of my mower. The left upper pivot of the cover looks the same as yours, but there seems to be an issue on the upper right: there seems to be a bolt head or nut where I have a round thing.

[Linked Image]
[Linked Image]

However I don't currently see why this would make your catcher fall off. I had problems when I first bought my Victa, with the catcher dropping off. The cause was always that the catcher had not dropped down all the way, to fully engage the vertical ridge on the top of the mower base. If it was not fully engaged, the catcher always fell off in a few feet of mowing.