Hoppe Cresent Knob, Euro Profile Cylinder.
Satin Nickel New Style with 90 degree Thumb Turn allows looking at thumb turn to see is door is locked.
Cylinder Size  55.5mm / 55.5mm ( 2.18 / 2.18 = 4-3/8 inch)  
Used with 2-3/4 to 3 inch Thick Doors depending on thickness of escutcheon plates

Take door thickness  3" x 25.4
3" X 25.4 = 76.2mm
Plus 1/2" on each side plates = 76.2 + 25.4 = 101.6mm
Divide by 2 = 50.8 minimum length - next size up 55/55

Available finishes: Satin Nickel, Rustic Umber, Matte Black

Article:
HOPPE Cylinder Installation and Rekeying