The carriage is configured for interchangeably holding pens of different sizes at the same position relative to a circuit that is mounted to the carriage. In a preferred embodiment, a black-ink pen or a relatively large color-ink pen may be carried by the carriage. The size of the carriage opening into which the pen is inserted is reduced by a spacer mechanism to accommodate the smaller pen, which spacer mechanism is retracted from the opening upon insertion of the larger pen.