What year was a gun made if the patent number is 2173576?

A patent number will not identify the gun but only some feature that was unique enough for a company to get exclusive rights to use it. It may have been used on several models by that company and they could have sold the right to use this feature to other companies, so it could be on hundreds of thousands of guns manufactured over a century or more. Winchester is still using patents issued in the 1890's on guns made today. Of all the things marked on your gun, the patent number is probably the least useful to identify the gun. Now, if that is a SERIAL number and you can tell us what manufacturer's name and model number are on the gun, we may be able to give you a year of manufacture.