97 HONG KONG WILL COME BACK TO CHINA .... What's this?

What I'm now studying is these Chinese commemorative sheet. The set contains five sheets and each has the number such as Hong Kong J97.111 5-1 but they are not stamps. I found there're  some examples in Chinese net auctions but there are no more information there. The set of five is sold at 15 RMB equal to around 2 USD. What is important for me is what it is.... If you know what it is, please give me some comment!