What make?
With the cover off operate the rail link arm. Does a mirror move?
No-arm slipping on the shaft, special reset of initial mirror position required.
Yes-mount camera on a tripod, point at an infinity target at least 5000 feet away, move rails in or out until images are in coincidence vertically then adjust the horizontal.

If the rangefinder will not make infinity observe the moveable mirror while moving the rail link arm and note which direction it moves with respect to the arm motion. Loosen the arm set nut/screw, run the rails in fully, move the movable mirror to its movement limit in the direction it moves when the arm is moved inward, then tight the arm set screw.

Post a picture of the rangefinder with the cover off if possible. Some Kalart have a small spring under the rear edge of the bottom prism frame that if off will prevent achieving infinity also.