Moving the slide locks on the back should let you remove the ground glass focusing plane and the viewing hood, not the Graflok back. There are two screws; one top center and one bottom center. Removing these two screws will allow you to remove the Graflok back, which is basically a frame with quick releases to allow easy swapping of sheet film back and roll film adapters.

If there is no focal plane shutter it is almost certainly a Crown, unless it is a Super Seed Graphic. You have a top mounted rangefinder so that's a possibility. The top slide lock for the lens board should say Crown Graphic or Speed Graphic. Try this link for a pretty good explanation of the different backs. http://www.xs4all.nl/~lommen9/holders/index.html

Good luck with the rails. Tuscon should have a camera shop that has someone familiar with press cameras.

Rick