If the mortise lock in your pictures is in fact a Russwin/Corbin lockset which it looks like there will be two different methods to remove the inside knob to get the spindle out so you can remove the lock case.
1. Look for a small hole probably in the knob itself that you need to push in the retainer then pull off the knob then need to use a spanner wrench if you have one or if the lock is going to be replaced anyway you can use channel lock pliers to unscrew the thimble which is the tubular looking thing behind the knob with the hole in it.
http://i1325.photobucket.com/albums/u62 ... Page82.jpg2 If you see no hole in the knob anyplace to depress the retainer then you need to unscrew the thimble from the knob itself again using a spanner wrench or channel lock pliers. Note this will unscrew clockwise as you are looking at it since it is unscrewing from the knob itself you will figure that out. Once the knob is off you can then unscrew the square parts to get the spindle out to remove the lock case.
http://i1325.photobucket.com/albums/u62 ... e84-02.jpgSpanner wrenches look similar to this below picture best example I could find quickly online.
http://www.motionpro.com/images/enlarged/08-0029.jpgYou can download the entire Russwin/Corbin Service and Parts Manual here. Ref page 82 and 84
http://extranet.assaabloydss.com/librar ... /45305.pdfI saved you time looking through this extensive manual to find the above pictures to get the knob off.
Where is the face plate that is missing from the edge of the lock case it should have the manufacture and model number on it.
Hope this helps you, most all these locks come off the same on commercial doors.
Let me know how you come out with this project.
You can PM me with questions you may have after examining the door again.
Richard