The problem you are having is a compatibility issue between two types of backs made for Graflex cameras. Your camera and holder don't match.
Could you please post a couple of photos of the other side of that roll film back? It should not have "Graphic" printed on it as it is NOT a Graphic holder, but a GRAFLEX holder. It will not attach securely to your camera as it was designed for the 2x3 RB Graflex SLR camera, not the Speed or Crown Graphics and the locking mechanism is different. You have to be very careful about which roll film holder you buy for a camera made by Graflex. There are two main types of backs for Graflex cameras. There is the Graphic back, of which the Graflok back is a later variant, and there is the older Graflex back, which is quite different. You also need to know what size camera the back is for, since roll film backs are also available for the bigger cameras and they share many common parts with the smaller backs. This has caused much confusion in the past and will undoubtedly continue well into the future. I would strongly suggest going to graflex.org and reading everything you can find about the various roll film backs that Graflex made.
BTW, if you should decide to sell the roll film back, I would be interested in discussing the possibility of purchasing it from you, via pm.