To move the saddle backwards or forwards, at the top of the straight post is a "pivoting arm" which pivots on the post top; this is the "coarse adjustment" to set the saddle layback (or forward). The saddle rail clamp pivots at the other end of the "pivoting arm". The pivot at the top of the post has teeth and notches, so that it ratchets around in sensible increments; the saddle rail clamp pivot is smooth and therefore continuously adjustable. (good)
I was staggered to find that both these pivots are clamped tight by a single pinch bolt. It follows that you can have both free or both tight, but not one free. (The head of the bolt bears on the saddle rail pivot clamp, the bolt is threaded into the clamp that bears on the seatpost top; it works because these are at opposite ends of a single bit of metal, the pivoting arm...its a long bolt which nips up both clamps. The serrated pivot at at the top of the seatpost needs more clearance than the smooth pivot at the saddle rail clamp (so you can't damage the serrations). As manufactured, this means that as you undo the common pinch bolt, the serrated pivot "lets go" before the smooth rail clamp pivot; you can't just back the pinch bolt off a bit to adjust the saddle angle.
It gets worse....the saddle rail clamp is not a single piece of metal which passes through the pivoting arm, there are left and right side clamps for the individual saddle rails; so in order to adjust the saddle angle you need both the pivoting arm (dual) pinch bolt and the saddle rail clamp pinch bolt to be loose....at which point the whole thing flaps about like a demented puppet with all its strings cut.
I think it would be much more sensible to have separate pinch bolts so that you could set the pivoting arm at your preferred setback and lock off that pivot; then you could work on the saddle tilt and slide the saddle rails through the saddle rail clamp without the whole thing coming loose.
As currently designed, the only adjustment you can make on its own (without everything loose) is to slide the saddle rails backwards and forwards through the rail clamp. It would be much better if saddle angle was also possible to adjust without everything coming loose. If I were to re-design it, I would have separate pinch bolts for the seatpost top pivot and the saddle clamp pivot so that you could lock those adjustments off individually; and I would have a solid saddle rail carrier going through the pivoting arm so you could adjust the saddle tilt and slide the saddle back and forth on its rails independently, with a pinch bolt each.
The manufacturers list saddle setback as 50mm. As usual, this is measured from the seatpost centre to the centre of the saddle rail clamp, but its the front of the clamp which limits saddle setback. The pivoting arm measures 60mm from the clamp at the top of the straight post to the saddle rail clamp (centres). Inexplicably, the clamp at the top of the straight post is offset forwards. Why would you do that? If it was offset backwards the offset would add to the setback rather than subtract from the setback.
My picture shows a Gilles Berthoud saddle mounted. A Brooks saddle won't mount as neatly as that; the shape of the saddle rails means the rails would foul the seatpost top if you tried to use the full layback angle.
The post is slack enough in my Longitude frame that I will have to "take steps" to ensure it doesn't slip down, but my vernier measures it at 27.2mm. I can't think of anything that would be sufficiently exactly manufactured to act as a "reference dimension" to check the vernier?
The saddle rail clamps, pivoting arm and seatpost top all appear to be well-made pressure die castings; the post top is possibly welded to a bit of tube and the weld dressed.
IMG_5353 by 531colin, on Flickr
