Based on the photos I found of the Mk3 accessory mount and ground glass panel in
this article, it looks like the back is Graflok compatible, at least to the level that it's easy to remove the ground glass, and doing so is likely to be required for most Graflok accessories to mount. I do know that on an actual Graflok (as opposed to "International", which is "compatible" but not identical) the focusing panel comes off easily and quickly and reinserts even more easily (I've now got two -- three counting the 2x3 one on my Century Graphic).
These backs are designed to be used by removing the focusing panel as needed. Most roll film backs, along with Grafmatics (the claim is that they'll slide under the focusing panel on a Graflex-made Graflok) and the new LomoGraflok and all Polaroid backs other than the 500/550, will expect you to remove the focusing panel.
This raises the obvious question of keeping it safe (broken ground glass is at best a gross inconvenience), but that's just part of the "dance" when using Graflok accessories.
