I currently use a singer 6X7 lever wind back, a graphmatic and some 2X3 filmholders on my Century Graphic. Are there any other kinds of roll film backs that will fit on the Century's graflock back? Someone told me an RB67 back will fit. Is this true? While the singer back works well, it seems rather flimsy, and I cringe when I see the price of 6X7 lever wind backs on ebay. Any ideas? Thanks. Rick.