Thanks for the response. Here is one of several sites you can use to look up dates for patents:
http://www.biddingtons.com/content/investingpatent.html
Here are a couple of pics of my ISSBL. Mine does not have, and appears never to have had, the non-slip pads on the steps. That is one thing that makes me think mine might be older.
If any one has one like mine I'd like to hear about it.
DW4