Ken,
The 15-91 was the only 15 with the potted motor.
There are two basic wiring systems used. One for cabinet machines where the controller cord is wired into the cord block and the power cord just plugs into the cord block.
The other is for portable versions. The controller is wired into the opposite side of the cord from the plug. They are set up different and you don't want to cross them up.
Here is the link to a blog where forum member Rain gives step by step instructions with pictures on how to rewire those machines.
{
http://vssmb.blogspot.com/search/label/rewiring }
Joe