If I'm lucky enough to buy a item with a patent number on it I have a handy chart that helps me identify what era it came from. A chart of patent numbers issued by year can be seen at http://www.uspto.gov/go/taf/issuyear.htm. This chart of patent numbers and dates is a great tool when you are trying to date antiques & collectibles. I keep a copy of one in my wallet and have used it at both antique auctions and estate sales.
The above picture is a packaged sash lock, the front packaging says Sold By Sears, Roebuck and CO. On the back of the package it says Pkg US Pat 3031072. To investigate this a little further you can go to Google Patent Search www.goggle.com/patent and type in a patent number and you'll get the description, the file date, the issue date, the inventor and the assignee. This particular number is not for the sash lock but for the packaging. Google patent search is another great tool for identifying antiques and collectibles.