what exact image is the basis for the 圭?
「圭」 does not come from a depiction of a tablet. That meaning of jade tablet is a semantic extension.





時期字體
字形
參考資料




—
A
—


商甲

乙6776合集11006


—
B
—


商玉璋

殷墟玉璋


西周金

師遽方彝集成9897


春秋金

旨於賜戈集成11310


戰國・秦石刻文

詛楚文


東漢隸

華山廟碑


楷






「圭」 (Baxter-Sagart OC: /*[k]ʷˤe/, jade implement) had two origins: A) the blade-head of a ceremonial jade dagger-axe 「戈」 (/*kʷˤaj/), and B) semantic 「士」 (bladed weapon &gt; soldier) with simultaneously semantic and phonetic 「戈」.

For reference, Shāng-era forms of 「戈」 inscribed with an emphasised blade-head:
商甲屯南2194　商金戈觶集成6054

Series A) doesn't survive in the modern writing system; for Series B), later on, 「士」 was doubled, and 「戈」 was omitted. The 「士」-shapes were eventually corrupted into 「土」, forming 「圭」.

References:

季旭昇《說文新證》
李學勤〈從兩條《花東》卜辭看殷禮〉
陳劍《說殷墟甲骨文中的「玉戚」》
小學堂

字形演變
金文


王輝《殷墟玉璋朱書「⿹戈士」字解》
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》