Today, Patty Swisher @pmswish posted a photo asking assistance John Danes @johndanes who forwarded to me and others. Since I get home later than you work, here are some thoughts.
The photo is with this tweet:
I guess I can start with what I see. It is an antique looking knobset and Patty wants to know the function of the plate below the handle. I also see many layers of paint and an oval in the same size as the centre of the plate to the lower right. Also the material under the plate does not seem to match the door colour. It may be a photograph issue, but the plate also seems to lack a screw to the left side. I am assuming the opening edge of the door is just to our left.
What I can not see and have to work from assumption is also interesting. Being old, it would be either a mortise block which is set into the door but they USUALLY have a keyway aligned below the knob or even older it can be a rim block which mount to the inside surface of the door. They would more likely have a keyway off centre.
I found a sample of this kind of lock on this page by Sheila Zeller at
Part way down is a nice surface lock by Jack in the Box. You can see its keyway if off to the side. Before standardization, the spacing was all over the place.
What would I like to know? Which side is the door edge and is any brand given? Is it a mortise block or surface block? What year was this building built -- or a best guess? Does our little plate have two screws and if you take them off is that a small bit of metal to block an old keyway?
I will guess my last question is the key. These old locks allowed the key from either side and let the wind whistle through the hole plus allowed people to peek in. Often, the plate would be taken off and a small cover slid or glued in to stop air and site. The keying was often warded and represents almost no security now so covering the hole is a good option in so many ways.
Do you have more photos?