Singer made several machines that had the Bentwood case. (The wooden dome case.) I don't remember when they moved from that case to the more modern cases. I own one machine that has it on it.) Usually the old singers in those cases are straight stitch. Some have reverse and some don't have reverse.
Here is a link to a site that might be of help in figuring out what model your machine is...
http://www.sandman-collectibles.com/...r-machines.htm
The machines are made of all medal parts and if kept lubed and oiled, they will last for years and years.
There is a Singer Vintage group on yahoo that is great about helping to find out about your machine and telling you where to find parts. Make sure to use their search function first so that you're not asking questions that have been asked and answered a thousand times already. If you have a chance, just go back and read many of the informative posts. Here is the link...
http://groups.yahoo.com/group/vintagesingers/
There is also another web page with directions on how to restore old sewing machines....
http://www.treadleon.net/
just scroll down to the restoring link.
Have fun! and depending on the machine itself, $40 is a good price.