I have a Kenmore 158.840. Not the same model as yours, but the same manufacturer - 158 means it was made by Jaguar/Maruzen in Japan.
The build quality of my machine is top notch, so if yours is anything like mine, you've got a keeper!

I think that the model number is the first two digits of the three digit number after the manufacturer code, so my machine is a model 84. Using this convention, you might want to search for a manual for a model 50.

Here's a link to the manual (free pdf download):

http://www.shopyourway.com/manuals/140166

Have fun! :)

Thank you very much