15-91.
The plate on the bed under the "singer" medallion will have a serial number on it. If you go here:
http://www.sewingshop.com/dateyourmachine.aspx you can look it up and find out when it was made.
Here's a 15-91 user manual:
http://www.ismacs.net/singer_sewing_...uals/15-91.pdf
A good look over to make sure it's mechanically sound and a couple drops of oil where indicated should be good for a test stitch...
edit: LOL, too slow typing it seems. Everyone beat me to the punch!
