Looks like China as only Mao stamps
Do you really think following 1968 China stamp sheet of Mao's Instruction is worth $15000?
We all know that FORGERY in China stamps is only way to collect because it was very hard to collect stamps before 1990 as communist have strong control on people what they should do and there was little communication inside China as well as outside China.
Following are some of the forgery stamps of china which as sold as REPLICA or COPY or REPRINT.
You can buy above forgery stamps and start selling for very very high profit.
Above $15 will get you $3000 - $15000.