Quote Originally Posted by Donald Qualls
It'd be an easy conversion, as such things go; masks that pivot on the pin rollers and store in the film compartments, and a little rotating mask in the viewfinder, plus a second window at the takeup edge of the frame gate.
It was certainly a factory job as every isolette L has it. But it is pretty much as you thought it would be. Except for the fact that it has 1 single red window for film numbers. I'v seen instructions on the net on how to advance properly for the smaller format using the dots between the numbers on the paper back.

It has a top plate specifically made for the camera with a lever to flip the viewfinder mask.