The Period brass deadbolt is a mortise cylinder deadbolt. The cylinders are screwed into the "donut" housing and held in place a set screw for both the inside and outside.
To access the set screws, remove the Deadbolt trim from the edge of the door,
locate a long 5/64 allen wrench and slide it into the exposed holes on the left and right of the bolt.
Loosen up the set screws
Unscrew the deadbolt cylinder by rotating the cylinder, a key blank or key that does not work the lock can be used
to help turn the cylinder