I found this cover in a dealerís box of world postal history. It is a Polish aerogramme sent registered from Zakopane to "Yuan-lin Formosa - China" on 25th November 1957. There is an unclear Polish transit mark of the same day (I think it is Warsaw). On reverse is a bilingual Peking receiving mark of 2nd December 1957. Somewhere, someone has added "No Reg", as well as an arrow pointing to Formosa and "Return", both in red ink.
My interpretation of the marks is that there was no transfer of registered mail from the mainland to Taiwan. Did ordinary airmail go by this route or was this cover transferred to Peking due to the odd form of address?