What is this door component-how does it go back in?

[NC_RVer]

I was switching out my existing E-Pro 19FBS door lock and I pulled out the original lock. In the process this plastic rectangle piece (see pictures below) fell out of the lock opening and I don't know how exactly it goes back in. Should the inside protrusion piece go on the right or left? Which side should be facing the lock?

I need to be more careful when I take things apart. Any help is appreciated.

John