You probably have a precanceled stamp with Manitowoc, Wisconsin in two lines between two bars. There are a number of different precancel styles of overprinting done to some of these stamps. They were intended to be used by large volume mailers and because they were precanceled, it avoided the need for the post office to cancel each and every piece of mail.
Precancels are a collecting specialty and there are literally thousands upon thousands of different overprints for various cities and towns across the US. The majority have little value, but they are a collectible to those specializing in them.
Does it look something like this?
