Although a bit off the Lange topic I'll share a few thoughts on the RD. The movement in your RD chrono perpetual is based on the Lemania movement used by PP and Breguet. RD calls it a RD5632. But the movement in the current monopusher is called an RD28. It looks similar to the Lemania based movement but is, in fact, thinner and smaller (10 3/4''' vs 12'''). RD claims the RD28 as an in-house creation, whatever that is worth. Both movements are very finely finished, at least looking at them through a loupe.
I've owned the RD for about 3 months now. In that time it has been extremely accurate (within +/- 1 sec/day on average) and the monopusher action is just about perfect with no jump or hesitation in the second hand.
Although it is completely different than the Datograph I enjoy it more. The RD is massive in weight, but sits comfortably on the wrist due to its curved design.
The Datograph is in a class by itself in many regards ... but there are some good options for those where the Dato just doesn't quite fit in.