In 1964 a man created a new product, the Postal Numismatic Cover or PNC. He also created a company, The 99 Company, to market them. Their first prototype was a envelope with a 1964 half affixed to it and a first day of issue cancelled stamp. Their actual marketable product had the half dollar in a die cut cardboard sleeve inside the envelope with the coin visible on both sides through mylar covered die cut windows in the envelope, and of course the first day of issue cancelled stamp. The 99 company lasted for many years and continued to make different PNC's with different coins or medals and first day of issue stamps. The company folded in the 1980's. Here is a link to a stamp roum that shows some examples of the 99 Companies PNC's https://www.stampcommunity.org/topic.asp?TOPIC_ID=35628