Your Grafmatics may have slots, but they won't fit a Graflex back. The graflex back is a different size, approx. 1/4 inch wider. That's why Graflex holders can't be used in 'standard' cameras.
So, yes, Graflex (RB,etc) cameras need Graflex holders in their Graflex backs. Graphic cameras with Graphic (spring) or Graflok backs use standard holders.
The slots in a Grafmatic are used to hold the unit into a Graflok back while it is operated so it doesn't pull away from the back of the camera causing a light leak.