I don't know whether the Chinese 612 backs have twin pressure plates; however, my Chinese 617 does, and it keeps the film as flat as a shit-carter's hat. Twin plates with strong-ish springs is a design that can't easily be bettered IMO.
In addition, the tension spring on the film spool means that there is good resistance when advancing the film, also assisting flatness.
I can GUESS at which are "the Chinese" backs, but it would be helpful to know which you've specifically used.
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~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller