IIRC from my buddy demonstrating his Galvin to me recently: the Galvin 2x3 back itself is not Universal or Graflok; it has a unique wide gape, bail back designed especially for accepting roll film backs UNDER the GG without it's removal. This means using standard 2x3 Graflok roll backs, at least those which won't beat up the GG too bad (for instance by having a badge in the wrong spot or by being too fat). I think Graflex/Singers work as do Mamiya RB types.