I'm not sure if this is the proper era for this question as it may have been added after it was built, but I figured I'd try here first just in case it was original!
I have just purchased a top floor unit built in the 1890s and there is a lever at the top of the stairs coming out of the wall. I know it's meant to open the front door, which is at the very bottom of the stairs, and I can hear the mechanism working near the front door when the lever is pulled but it doesn't unlock the door anymore.
I'd like to fix this, but I don't know what it's called! Can anyone help?
This is a very poor photo of the lever, as I didn't think to take a photo of it the last time I was in the unit.

Any help would be greatly appreciated!
Thanks
Stella