I've long been interested in building one of these and in fact have one of the aluminum versions hanging in the shop now that I've not yet gotten around too. The design is another from Horst Leitner (see below) and essentially the same as the AMP B3, but with the shock mounting location moved from under the top tube to behind the seat tube. There are aftermarket air shocks (Risse) available and the quality is quite good enough to justify putting a bit of effort into building it as a lightweight FS XC rig.