I now have a 55 key Lachenal Crane in this configuration. Purchased about 6 months ago in standard layout (my first duet, and first concertina aside from some very limited trying out of an almost unplayable English), and after a few months began the transition, then sent it off to a shop for refinement, and received it back just a few days ago. It's unidirectional, with lowest C keys on both sides in the index finger positions. Am very positive about it. During the beginning of the reconfiguration I had been very curious about whether a 6 CC as mentioned by Little John would be a better way to go, and had done some tabletop practicing, but decided to go with what I already had, and in any case a Maccann might be more problematic to convert, though I've never seen one so of course don't know.