The Yale Locks YALPM560PB25 PM560 Hi-Security BS 5 Lever Mortice Sashlock 68mm 3 inch comes in the following:
Finish: Polished Brass
Case Size: 81mm x 109.6mm
Backset: 57mm This is the measurement from the edge of the door to the centre of the keyhole.
Centre: 57mm This is the measurement from the centre of the follower, which operates the handles, to the centre of the keyhole. In the UK this is commonly 57mm.