I'd like to summarize what I've learned in this thread

> Aperture used in the f/ number calculation it's not the physical hole in the iris, it's the effective aperture "seen" throught the front element side.

> Taking lenses have the f?number for infinite focus, as we give bellows extension to focus close we use a compensantion factor for the modified aperture.

> "Aperture in mm" in a repro lens allows to calculate aperture directly with using and bellows extension and the mm indicated in the aperture scale, those mm are not about the actual hole (I guess), but about the effective hole seen from the front element side. As those effective mm depends on the front glass then that scale is placed in the front casket, if we replace another front casket then we have the right readings in the scale...