Door lock mechanism operation help

[Jonnydaz53]

Hi guys i have the same as whats in the photo, as you can see the lock on the left has a extra metal slider than the other one on the right

The question is , what does that slider do and should i have one on the right side .

Thanks in advance

Regards

John

Untitled by john Di Benedetto, on Flickr

[pors1968]

Production year are stamped.

[jimtweet]

Pretty sure that prevents you from locking door while door is open. In other words door must be closed and key inserted to lock, in other words a “don’t lock your keys in the car lever thingy”