Shanghai brand 312 single-sided blue carbon paper 12K handwritten copy paper blue printing paper 12 open single-sided printing and dyeing paper large A4 specifications 21 5*33cm 100 sheets of Shanghai blue typing carbon paper 12K handwritten copy paper blue printing paper 12 open single-sided printing and dyeing paper large A4 specifications 21 5 * 33cm 100 sheets of Shanghai Blue typing carbon paper