The Yamaha SW396 is a two-stroke snowmobile produced by Yamaha. This snowmobile was first introduced on the American market in 1970. The European market would see this machine as the SW400B Europe in 1972. This machine was the predecessor of the SW 643 and the later SW440.
It was the first Yamaha snowmobile to have a reverse gear. http://www.yamahapart.com/page/yamahasnowmobilehistory
The engine of the SW396 and SW400 was an 396 cm² twin-cylinder that produced 27 horsepower at 5500 rpm.
References
Yamaha SW396 Wikipedia(Text) CC BY-SA