Conclusion: It is "Heaven and Earth Xuanhuang"
Reason: As mentioned earlier, it is a matter of Qing Dynasty to avoid the taboo of Qing Xuanye. Also, the widely circulated "Yuanhuang" must be a taboo to avoid the emperor, rather than the "name" of some literati (in ancient times, taboos were generally taboo to avoid the emperor and one's ancestors, and no one would avoid others. (name, have you ever seen "白" that avoids Li Bai)?
We can see from Zhao Ji's Shou Jin style "Thousand Character Essay" that Zhao avoided the taboos of his ancestors and changed many characters, such as "Xuan", "Yin", "Shu" and "Heng".
There is something similar. The opening sentence of "Preface to Prince Teng's Pavilion" is "Yuzhang's old county". Later, to avoid the taboo of Tang Dynasty Emperor Li Yu, it was changed to "Nanchang's old county". This is a continuation of many calligraphy works of later generations. "Nanchang old county".