At present, the simplified word "Bei" has actually existed since ancient times, and it appears as a vulgar writing form of cursive script. The formation time is roughly the Eastern Han and Three Kingdoms period.
For example:
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-This is the writing method of the calligrapher's statue in Wu Dong in Three Kingdoms, which is no different from simplified characters.
-This "shell" was written by Zhao Gou, the emperor of the Southern Song Dynasty.