Medeco mortise cylinders threads on the outside of the shell and are designed to be screwed into other manufacturer's mortise lock cases. A cam on the back of the cylinder operates the lock mechanism and a set screw holds the mortise cylinder in place
This cylinder is for narrow stile glass door lock and many mortise lock.
This new Medeco M3 keyway cylinder.
Medeco M3 Mortise Cylinders provide a high degree of security by incorporating special design features inside each cylinder. Medeco Mortise Cylinders offer superior pick resistance, are bump proof, offer excellent resistance against drilling and other forced attacks, large number of combinations and a utility patent to protect against unauthorized key duplication.
(Standard cam) 
(Adams rite cam)