Why isn't the 20 15 panda coin engraved?

This happens to be my major. Let me tell you something about it.

In 20 14, the state promulgated a unified unit of measurement for precious metal commemorative coins, changing ounces into grams. When this document was issued, 20 15 panda coins were almost used up. On the one hand, the mold and packaging have been completed, on the other hand, if it is modified, it will affect the release progress. So I simply don't mark the gram weight on the commemorative coin. Therefore, the 20 14 version became the last version marked with ounces, and the 20 15 version became the version without grams. Starting from the 20 16 version, the number of grams shall be uniformly marked. The third edition has a high collection value because of the special marking form of measurement units.