China, 718-732 AD., Tang Dynasty, middle Kai Yuan type, 1 Cash, Hartill 14.4 u var.

China, Tang Dynasty (618-907), Tang Dynasty, middle Kai Yuan type, issued ca. 718-732 AD.,
1 Cash (25 mm / 3,79 g), cast, six-pointed center hole ("turtle shell hole"),
Obv.: 開 元 通 寶 , Kai Yuan Tong Bao ("The Currency of the K'ai-yuan Period" or "The Inaugural Currency") , top-beneath-right-left of central hole. The "jing" component of "Kai" touches the hole, the top stroke of "Yuan" is longer, left shoulder on "Yuan", the two stokes in the middle of "Bao" do not touch the verticals, distinct vertical parallel lines in the top component of "Bao".
Rev.: top: crescent at angle, over six-pointed center hole (a "turtle shell hole"),
Hartill 14.4 u var. (six-pointed center hole) (rarity of the normal type 14.4: 13, very common) .

Kai Yuan meant “to open a new beginning”, and it did indeed set new standards for casting, legend, style and shape, that were followed in almost all later dynasties. The legend of Tang coins consisted of four characters read top-bottom-right-left.

A possible explanation for the non-square center hole based on info on http://primaltrek.com/flowerhole.html :

Most cast Chinese coins retained a familiar round shape with a square hole in the middle for more than two millennia. A small number of these coins, however, were released into circulation having holes with a more than four-sided square hole, they would have holes with"eight sides" (octagon) and, because their shape resembled flowers, the Chinese referred to them with such names as flower hole, rosette hole, or chestnut hole. Other coins would have holes with "six sides" (hexagon) and were said to have "turtle shell" holes. Westerners sometimes refer to these coins as having "star" holes.

For good or ill a significant number of them passed quality control from the Qin and early Western Han Dynasties beginning in 14 AD. during the reign of Wang Mang til the production of flower-holed coins declined sharply starting about the time of the reign of Emperor Xiao Zong (1163 - 1190 AD) of the Southern Song.
The large number of these coins that found their way into circulation during particularly the Tang and Song dynasties, which were renown for the high standards and quality of their coinage, would indicate that the flower hole was an intentional embellishment having important significance. Theories are that they may have identified a location where or time when the coins were cast or a symbolic function.
The appearance of flower holes on Chinese cash coins must have been intentional. They must have been created after the coins were removed from the molds during a final chiseling process. The coins were first broken off of cast "coin trees" (qianshu). Then, any excess metal that may have accidentally flowed into the center hole of the coin had to be removed with either a chisel for file. Finally, the coins were stacked onto a long metal rod. The rod was square so that once the coins, with their square center holes, were stacked onto it they could not rotate. The workers could then use a file to remove any metal sprue from the rims to make sure the coins were round.

Maybe the square metal rod whrere the coins were stacked onto in some cases seved as an internal file creating two or four additional sides to the center hole - intentionally or not.