Welcome! Can you post a picture of the machine? It will go a long way in helping you figure out what you have. Meanwhile this might help with a ballpark date. As a rough guide, patents were good for about 7 years. I think there are exceptions to that but it's a start. Here's a list of patent #'s and what years they were issued:
http://www.uspto.gov/web/offices/ac/...f/issuyear.htm
You know the machine was made after the last patent date so if you figure from then until 7 years later you're probably pretty close to right. Some manufacturers left patents on after they expired so it's not an exact thing.
Rodney