US Stamps - F - Registration Stamp
A registration stamp was not for postal use. It was affixed to an envelope to show that a person had paid the fee to have their mail registered. Registering your mail guaranteed special handling of what you were sending. This US stamp (Scott# F1) was eventually discontinued due to confusion over how to use the stamp properly. It can still be used however to pay for the first 10¢ of having your mail registered, but now any stamp can be used to pay for this.