That is a Graflex back bag magazine. I had one for my Graflex Super D that held 12 sheets. The Graflex back holders have slots milled into the face near the open end, and along the sides. You can see the slot near the bag end of the mag in your photo. They are also slightly wider than the standard Graphic holders.
Standard holders have a ridge for a light trap and no slots along the sides.
The Graflex holders are scarce however. You also probably won't be able to find roll holders, and I know the Polaroids won't fit. The Graflex holders are scarce however.