This is a fixture to line up the mounting holes for an ASM mortise lock (faceplate dimensions of 8"x1-1/4").
Door manufacturers will usually not drill holes smaller than 1/8" so the pilot holes for the lock body are not drilled or they only put a small dimple and it is rarely in the correct location. If you put the lock case in the door and use that as a guide, the faceplate will usually be tight in one direction or another so this solves that simply. Its not the first one of these to exist but its the one I can print at home and not be upset when I lose it.
It fits into the existing cut out for the lock faceplate on the scalp of the door. Use a #10 vix bit to drill through the top and bottom hole into the door.

I print them in PETG, 0.16mm resolution and at least 6 base layers and at least 40% infill. 6 or more wall layers because the pilot hole locations wear out eventually and this buys you some time before you break through the walls and into the infill.

Each time I print one of these, I make a tweak here or there so this is a work in progress and Ill upload any significant changes if anyone out there cares.