So using the square format mask won't give you more exposures per roll?
And the ground glass is rectangular vertical?
Not normally, but I haven't tried to 'under advance' the film to get more shots in yet. I'm not sure if it would even work, but next time I put the square mask in I'll give it a try. In normal operation you can only advance the film winder a certain distance before it stops and won't advance anymore until the shutter is released. However, you can release the shutter at any time which makes it possible to do double and multiple exposures. If you 'under wound' and then released the shutter I'm not sure if it would then reset the winding distance or not... you would also have to guesstimate how far to wind to prevent overlapped square frames if it did work. (I'm not sure if any of this is making sense, I just woke up and should probably have a coffee before trying to reply to anything)
The ground glass isn't really a ground glass like those found in higher end TLR's - it does have guiding lines to frame your shots depending on what mask you are using however and shooting with the 36 x 24 mask will give you portrait aspect (vertical) exposures.
(edited to correct mask size it's 36 x 24
not 36 x 26)