Why did you assume that the distance would be 5.9 inches? Just because that is the lens focal length in no way means that it would also be the flange focal distance. You need to go to the Rodenstock web site and see if they have the specifications for this lens. If not, then you might need to contact Rodenstock by phone or mail and get the specs. Depending on the optical configuration of a lens, it not unusual for this type of variation.