Welcome, 18% grey - you certainly are a card!
Just in case you haven't found out yet - make sure you know which model of RB67 you have (Pro, Pro S or Pro SD). The backs are different for each model, because Mamiya changed ("improved") the system of interlocks with each model. Trying to fit the wrong back will just damage it or the body, and probably it won't work anyway - they aren't backward or forward compatible.
With medium-format, I've found that used-equipment dealers often assume their customers know all about the technical side already. So when they list "an RB67" that's just the naked camera body - with the revolving mount at the back, but no film magazine or lens (and, unless they specify, usually no finder either: just the bare ground-glass screen). The mount is officially an integral part of the body, and should be sold as part of it - though, because they sometimes go wrong (and because they can usefully be fitted to other cameras), they are available as separate items too.