In olden days Ratners and Milners were drilled open using templates aligned with the keyhole and which gave the exact positions of the six fixing screws which passed through the hard plate and were only half an inch from the surface of the door.
This technique only applied to the List 1 as the grades above had hardened door surfaces.
Perhaps this illustration will allow you to make a template although it is a much later lock. i.e. 1914.
