The 6x7 backs do go quickly! A few years back there were lots around for peanuts as the pros dropped their RB67's onto the market and went digital. Now a lot of people who could never afford such a magnificant camera have them and demand has picked up again. A brief note on the backs: Pro ones, pretty old now, no interlocks so you have to be careful over double exposure. ProS, most common, usually need the foam seals replacing, it's a easy (but messy) DIY job of about an hour, materials and instructions are available on the web. ProSD have no foam seals, so that's not a job you have to do, plus have a little place to store the darkslide when it's out so you don't use it. Beware, there are a lot of 220 format ones out there as 220 format film is dwindling (120 gives ten shots per roll, 220 gives 20 shots) but as the film thickness is different since 220 doesn't have the backing paper, it's not a good idea to use 120 film in a 220 back). 220 ones are cheap and easy to find, 120 6x7 less so. happy hunting!