Is a MOUNTING PLATE what I need?

I'm not sure what handle you have now, but the 82-240 is designed to work with our 82-080 handle set. The 82-108 mounting plate does with with this handle, but there's no guarantee that it's what you need. I'd like some photos of your existing handle set so that we can say for sure.

Huh, this is a weird one. The handle set on your door doesn't match the mortise lock. Your mortise is very clearly our 82-238, which is designed to work with handle sets like our 82-080 (and by relation, our 82-240 thumb latch, which actually comes included with the 82-080 handle set).

However your handle set is an entirely different style compared to the 82-080. It looks like a more typical set like the 82-069 or 82-079. These are designed to work for different doors with different mortise locks. 

I don't know for sure, but I would wager that someone in the past repaired this door with the wrong hardware. Either they used the wrong handle or they used the wrong mortise. If the mortise works fine, I'd say that the handle is what's wrong here.

I think your best bet is to use our 82-080 handle set to repair your door.

Our 82-108 mounting plate probably isn't compatible with your existing handle set. It's not a universal mounting plate for any ol' door, it's only meant to work with one handle and one handle only, which is the 82-080. 

The 82-080 in fact comes with the 82-108 included already; we only sell it separately because sometimes it breaks before the handle does and we want to have an option for customers in that situation. Otherwise, I don't see it working for anything else.