Is it one of the Langs Keyed S/Locks?..where you open up that flap/hinge & insert some kind of small key to remove the lockplate?
Is it yours...& do you have the key!!!
Is the lever shown fully seated, or not quite seated?
I've seen pics of Lang Key SLs, but the flap seemed a bit bigger and was more forward than this one.
That looks to be an indicator on that side...if there is no screw on other lockplate I would think thats what its 4
Could be way off though...sure looks pretty
cheers
franc