'Empty' coin

I suppose that this 'empty' coin was used for weight and volume filling in the coin's strings. Usualy coin string had had 1000 1 cash coins but if somebody want to give less coins to somebody, he used such 'empty' coins in the string.

There was real coin with empty obverse and reverse in the Chinese history. It was issued during Wang Mang interregnum (9-23 AD) but have large center square hole 10 mm.

This coin looked like being worn out by someone intentionally, and this must be done long time ago. If you look at the upper image of the left image, at it's 9:00 direction, you will find some scripts still left, even though not readable. I don't know the reason for such a person weared out the coin, but in ancient China sometimes inscription of a coin was not so important as it's weight and content, i.e. copper, contained in the coin. I think this coin might still be put into circulation in the past, if the recipient had no objection. I have seen few cash seriously worn out, and almost can not be recognized. The difference and similarity among them and shown coin, I think, is matter of degree, and no one will cast "empty coin" for circulation on purpose.

Your explaination seems correct. It is not a plate, and the hole is cut in a fashion unlike normal cash coins. Notice they have slight curves on each edge. Since the metal is what really counts in those days, this coin probably is made of lesser materials, or used as a local private token.