Whose calligraphy is this?

The photo is a work of calligraphy and seal. The contents of the calligraphy and seal are Chairman Mao Zedong's "Lang Tao Sha Beidaihe".

The author is signed as "Da Jun", and I don't know his last name. The seal in the lower left corner may be a famous seal, but unfortunately I can't see it clearly.

Hope this helps.