It is a Graflok back. The film back seems to fit securely and snug, minus its ability to shift out on the left side.
Sorry for the crappy cell phone pics. I was being lazy :/
As I thought, the roll film holder in your photo is for the Graflex camera back. The Graflex back was on the SLR's and some of the older Graphics (pre-anniversary models, I think). Note the vertical groove in both the camera and the roll film holder. The proper holder has a vertical ridge instead of the groove.
As I mentioned if you and slip in something that will engage both grooves, you should be able to use the roll film holder on your camera, but that is a bit of a kludge.
Oops
Added: Sorry, I believe the grooves are on the opposite ends so you can not actually fit something between the grooves. I am not even sure the backs have the same film registar distance.
The problem comes because over the years Graflex (the company) made cameras with three different backs. The Graphic (spring) back which is held in place by screws. The Graflex back which was similar to the later Graflok back but different (this is what your film holder is for), and the Graflok (also called and International or Universal back).
Glaflex and Graphic film holders usually sell for 1/2 or less what Graflok film holders do. The reason for that is because most reasonably modern large format cameras can use the Graflok film holders.
From the photos, your camera very definitely has a Graflok back. That is good because some of the earliest Pacemaker 23 cameras did not. All the other Pacemaker Graphics came with the Graflok back. While the later lever wind roll holders are all Graflok as far as I know, knob wind backs may be any of the three so you have to be careful about what you buy.