How many sets of cherish mineral resources stamps are there?

On December 9, 1978, the Ministry of Posts and Telecommunications issued a set of special "Mining Development" stamps related to China's mining development, numbered T20, a complete set of 4 stamps.

After the founding of the People's Republic of China, large-scale geological exploration combined with mass mineral prospecting has proven that China's mineral resources are very rich and complete in variety. There are more than 150 kinds of known minerals in the world. By the end of 1984, China had proven reserves of 136 kinds, of which more than 20 kinds rank among the best in the world. While prospecting, there are also plans to gradually establish mining industrial bases for ferrous metals, coal, petroleum, non-ferrous metals, rare elements, etc., striving to meet the needs of agriculture, industry, national defense and scientific and technological modernization.