Welcome to the world of RB67, Hugo.
Here is a PDF file
http://www.mamiyaleaf.com/assets/files/documentation/RB67_Pro_SD_v7.pdf.
As already stated, there are three versions of the RB67. They are: Pro, Pro S, and the Pro SD. Basically, not to get too confused the Pro S is an update to the Pro, where as the Pro SD also being an upgrade to the Pro S, it has a slightly LARGER opening for two later lenses that will not fit the two earlier models. Those lenses are the 75mm Tilt/Shift lens, and the 500mm Telephoto lens.
Determine which model you have to make sure you understand the lens issues. Not a big thing at all, as for the most part Mamiya was excellent about keeping things a bit backward compatible as they moved forward.
If by chance you have the Pro SD model with the LARGER front opening there is an adapter ring or sleeve you can use on the lenses that make a tighter fit on the LARGER opening.