IMHO this is a typical crude fake with the red colour of China mainland much darker than with original
And just for fun .... what about the air mail label?
8 f. was the basic weight letter inland letter rate in 1968 (1955 to 1990 rate period in PR China).
Inland air mail required extra 2 f. per 10g (1958 to 1990 rate period in PR China).
rate table
https://www.chinastampsociety.org/f...able-PRC.pdfSo my guess is: somebody removed a 10 f. small size definitive from an otherwise OK inland airmail cover and mounted the large fake over it, covering remainders of the original postmark.
The "postmark" seen now is also a fake. These are easily to procure and sold outside large stamp fairs with complete sets of date inserts. Nobody cares.
The clumsy faker did not care for the air mail label.