What is this?

[phil]

my theory is that the door first lived in another location and was turned about. Maybe it was part of a pair of french doors and the spring loaded "ball" was supposed to fall into a detent to keep the door closed. I cant' quite see but it appears that the hinges are not chiseled out as normal, so that made me think they were installed by a homeowner, rather than being fitted properly. maybe origionally the door never had a knob hole but just clicked into place, and there was no real need for a lock if it separated an internal porch or something. perhaps closed either way by double acting spring hinges so then the regular type of striker plate wouldn't really work.

that door may have swung from the other side and been modified to its current location.

[Texas_Ranger]

Aren't ball catches more likely to be on the door rather than the jamb? It seems a bit low down to me too, below the bottom hinge as far as I can tell from the pictures. Unscrew it and see if it's a switch!

[phil]

Is this thing on the door jamb, or the edge of the door?

[mjt]

Looking at the photos, the door is the red part. The switch is in the jamb.

[phil]

now its bugging me. maybe an alarm, or a switch for the trap door to the dungeon?

It makes me remember when I had a light in the closet to try to grow pot plants and a switch so it turned off when you opened the door. I was probably 15or 16 or so. mom didn't think it was very safe. she said go put them outside. The mice ate the little sprouts. So much for my master plan.