I found this NOS original seat in perfect condition on ebay. This is the correct one for a 1970 model. On my bike a previous owner mounted a seat of a 1971-1972 model which looks more modern and also is not so heavy. The 1970 seat is old school style with metal springs.
![seat-nice](http://restorations.aermacchi-world.de/wp-content/uploads/2013/04/seat-nice-150x150.jpg)