To Add to John's comments

Excellent analysis John. I think that your post deserves archiving since this issue comes up so often. You forgot to add a couple of items. First, Patek changes the balance to its free-sprung Gyromax system--a very important change. Additionally it must do something to the barrel as the power reserve jumps from 48 hours on the "stock" Lemania to 60 hours on the Patek version. Also, the return to zero spring is different from the basic Lemania. At the end of the day, what really carries over is the some of the gearing, the shape of return to zero hammer and the minute counter mechanism. I, for one, have no difficulty at all in thinking of the 27-70 as an in house movement. Jeff