Do you sell these black plastic latches?

Hello! Sounds like you need a new keeper for your mortise lock. Now, we don't sell a keeper that matches what I see in your second photo here, but I think we can still help. 

Reviewing the design of your mortise, it looks like an older version of our 82-268B. If I'm right then we sell two compatible keepers: our 89-042 and 89-030. These two parts may work on your door, though you may need to drill new holes for them to align properly. Look over their specs and see what you think!

Dimensions for 89-042

Alternate image for 89-030