I eventually thought sod it and got one of the cheap Chinese (isn't everything now?) backs off E-Bay. It is for 5x4. 6x12 only - not a Shen Hao - it dosn't look that sophisticated!. It it pretty crude and simply made, but as long as the darkslide is not pulled all the way out during exposure it works really well. with no fogging, scratching etc, and as I usually use 5x4 I dont mind the basic-ness of it at all. It is made by GUOHUA and it's simplicity might frighten some!