I didn't realize that there was a dual use back. I was of the impression you put an insert into the sheet film back. Whatever it is an extremely rare thing and I have never seen one.
The sheet film backs and film holders are one of those ebay items that no one is sure the value. So you see them listed with widely varying prices from hundreds of dollars to nearly nothing. I was patient when I got mine and waited for those listed with low starting price and no reserve. I found that I usually had no one bidding against me.
If you wanted to communicate with a guy who knows as much as it is possible to know about the Rollei cameras and systems you could join the Yahoo Rollei User Group and put the question on there. A man named Carlos prides himself in knowing all the answers. He will know about the back.
http://groups.yahoo.com/group/rolleiusers/

Dennis