Mortise lock replacement

I think I see what you mean. The slot on the side of the mortise that takes the key lock is a lot thicker on yours than it is on ours. Can you show me a photo of the handle set that interacts with this mortise? I want to see the whole thing, but especially the thumb lever that goes into it. That may shed some light on what you have and why it's designed this way.

Hey there, hopefully I can help. I do believe that our 82-218 mortise lock should work for your door. We couldn't find one with a larger slot, but it may not matter. My only thought would be the thumb turn lever in your existing handle might be a larger size to accommodate it, however if you find that the turn lever tail is too thick then you could always just file it down a little.