A simple trick for aligning a rangefinder on the bench, without relying on an exterior image...
Take a piece of white foamcore and scribe two crosses on it, one with a horizontal line and a vertical line, the other at 45°.
The trick is to space the centers of the crosses EXACTLY equal to the center-to-center distance between the two rangefinder windows.
Set the camera up on a tripod, focus set at infinity. Support the X target in front of the camera, parallel to the film plane, and well illuminated.
The actual distance from the camera to the target does NOT matter.
Align the X target so the direct (non-rotating) rangefinder view is centered on one cross.
Adjust the rangefinder infinity setting until the centers of the two crosses overlap horizontally. A vertical adjustment can be set if available.
Shorter ranges should be focused correctly if the rangefinder has only an infinity adjustment.
If it has adjustments for shorter distances (like the Kalart), additional steps are needed.
This technique will work with any rangefinder that uses a movable mirror.