Isn't the Olympus Pen a Half frame SLR?
You can find more information about half frame cameras here:
http://www.cameraquest.com/classics.htm. I know that the russians made some half frame cameras too.
IMO, half frame cameras arn't worth the time. It's harder to scan, produces a smaller neg (and thus, less detail on the picture) and may run into some problems when you process. Not worth the time. Besides, why settle for half when you can get the full 35mm size?
Samuel