I am trying to figure out exactly what the difference between a Graflex back and a Graflok back is.

Why is a Graflex "not a desirable feature on a Graphic,"?
(-as mentioned at: http://www.graflex.org/speed-graphic/features.html#GraflexBack )

Graflex made 3 different backs, that being Graflex, Graphic (Spring Back) and Graflok. The graflex back will not accept the moderm film holders. It can be identified by its 1 sliding bar at the top. The graphic has a spring loaded back/focus screen and may or may not have a removable hood, the older ones had a pop up hood with side curtains. The Grafloc has a sliding bar at the top and bottom and will accept a removable focus/plate holder and at least in the 2X3 cameras rollfilm backs of 8,10,12 or 24 exposure. ie. 120 film or 220 film on certain backs. The graflok is the most desirable, the Spring Back is next and the Graflex is last.

Spring or grpahic backs had been around for quite a while when F&S introduced the first Graflex SLR. They needed something different to use their Graphic Film/Plate magazine that could be up to 1.5" thick and could fit under a spring back. Since the SLR didn't need a ground glass, they invented the Graflex back.

While most of the Graflex backs used one slider on top, later Pacemaker models had two. The largest indicator of a Graflex back is the necessity to remove the ground glass before inserting a film holder.

Up until the late 30s, this was a desirable back since it was so verisitle, it could handle sheet film, the magazines and even a roll back (that was 5 inches wide).

The graflex back fell out of favor when roll backs and the Graflock back was introduced. Now you can do everything the Graflex back did and have the ground glass accessable.