The regulation was that you could only drive it 30 km/h, only keep the two first gears and no suspension on the rear axle. In the mid seventies you couldn´t build a new one because they began to go very fast...
It came out new regulations to build a so called A-tractor to pull big wagons etc. They were not allowed to go any faster than 30 km/h at the highest gear at maximum hp and tourqe.
You can drive it when youre 16 years old. It was often built on heavily customized pickups or modified stationwagons (pickup built because you have to have a flatbed on it) with a frame.
Mine did only 30 km/h when I bought it, a Mazda -77 pick-up. I had it like that for one week then I threw the second gearbox (that made it go slow) in the trashcan.
Now it went 140 km/h insted... But with small inventions incase the police would show up.