What kind of conditions has the system been stored in? Any dampness? If so then the rail guides may have some aluminum alloy corrosion making the fit even tighter.
The probable solution is to remove the 853A case cover. If you have the bail handle, then remove the plastic cover from each side, they snap on/off. Next remove the center screw from each pivot point. They handle can now be spread apart and removed from the 853A. Next, remove the rear feet both sides and the rear cover screws top and bottom center of the cover, if any. Now you can remove the cover by sliding backwards. You can see the plugin latch and check the rails for any corrosion. If necessary you can lever the plugin out of the mainframe if the latch is released.
Don Bitters
