They're not common, but they're out there. The price usually reflects their scarcity; nice condition will usually run about $150. To the best of my knowledge, no mask was ever made. It would not have been necessary, really, since the ground glass is already ruled vertically and horizontally for a 6x4.5 crop. I find it difficult to compose with those rather faint lines, however, so I masked the ground glass with black graphic arts tape that doesn't leave a residue. Of course, that body is used as a dedicated 645 body; I'm embarrassed to say that I have two (!) other bodies to shoot standard 6x6. Hey, S2 bodies are still cheap!