I have both flavors of Graflex-made roll holder, for Graflok and Graflex backs. Erik and greywolf are right. That's a roll holder made to fit a Graflex SLR.

Interestingly, early 120 film roll holders for 2x3 and possibly larger Graflexes have both the slot and the groove, will fit both Graflex and Graflok backs. I have one.