I haven't been on the forum for a long time, som I'm a bit late. but I hope this can be useful.
My reprint of the Zeiss Ikon 1936 catalouge says that 668/2 is for the Maximar. 728/2 is for Zeiss Ideal. It's a pop-off back, quite different from the 668/2. It will only fit the Ideal and Ica Minimal I think.
I am the lucky owner of a 9x12cm Ideal. I find the fold out hood of the ordinary groud glass back getting in the way when I use a dark cloth, so I recently made a new ground glass back from my film pack adapter. I just pulled out the hinge-pin with a pair of pliers and removed the lid. Then I glued a piece of ground glass in place with some silicone. Couldn't be easier. Next step is to mount a fresnel lens. The best thing is that the film pack adapter is not permanently damaged, so it can be easily restored.
Good luck!