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[[Zerollo]] produced substantially only one model, albeit in a considerable number of variants, called [[Zerollo|Duo Color]] precisely by its characteristics of being made by two internal separate pens allowing to write in two different colors. The mechanism, which incorporates the technology used on the classic [[safety]] pens is extremely sophisticated and bears close tolerances, which makes the pen rather weak in terms of mechanical stability. Through an ingenious screw system, the two pens, which are separated by a metal plate attached to the bottom that makes them rotate, are sliding in opposite directions by means of grooves made ​​in the body of the pen, and brought alternately inside and outside the body according to the direction in which you rotate the cap.

''Zerollo'' produced substantially only one model, albeit in a considerable number of variants, called [[Duo Color]] precisely by its characteristics of being made by two internal separate pens allowing to write in two different colors. The mechanism, which incorporates the technology used on the classic [[safety]] pens is extremely sophisticated and bears close tolerances, which makes the pen rather weak in terms of mechanical stability. Through an ingenious screw system, the two pens, which are separated by a metal plate attached to the bottom that makes them rotate, are sliding in opposite directions by means of grooves made ​​in the body of the pen, and brought alternately inside and outside the body according to the direction in which you rotate the cap.