They're manufactured by Plasser & Theurer in Linz, Austria.
I have a video of Queensland Rail's Plasser ballast tamper in action, I will upload it and post it here. It very clearly shows the unit in operation on a section of track.
QR's ballast tamper runs on 3ft 6in gauge track meaning that the machines can be customised a bit, so I am surprised that they made it to the States with their buffers intact!
Kieran.